The Fog of Ages

"Treebeard: I don't suppose you've seen Entwives in the Shire? Pippin: [shakes head] What do they look like? Treebeard: I don't remember now."

- The Two Towers

So, it has been clarified, immortality sucks. You see all your friends and loved ones die off, you have to constantly come up with new forms of ID, and if you don't keep abreast of mortal matters, you're going to find yourself completely irrelevant in a century or two. On the plus side, you're a living witness to history. You could have been alive at the fall of the Berlin Wall, the treaty at Appomatox Court House, or even the signing of the Declaration of Independence. And you'll be able to recall that as long as you live, right?

Well, hold on a second... see, one could argue that there's only so much space the brain can hold, like a video cassette. Which means that, if you live long enough, your brain's going to start recording over itself. You might forget where you were born, what your parents looked like, hell, maybe even what your real name is. And it'll be lost forever to the sands of time.

Sucks, doesn't it?

Unless, of course, the immortal character in question records everything in a diary/volume of books or something, but how often does this happen?

A common partial aversion is to have the immortals friends and family be the only thing they still can remember. The tragedies, the wonderful days, the good times and the bad may all blur together after a while, but they can still remember those faces.

Not to be confused with Shrouded in Myth.

Anime and Manga

 * Killy of Blame! is a cyborg who has existed probably as long as the strange machine world of the manga. He is so old that he has forgotten even that he is a cyborg. At least, it seems so. It's hard to tell.
 * Probably exacerbated by the number of times he gets shot in the face.
 * The Abh touch upon this in Crest of the Stars and its sequels. They live for between 200 and 250 years and their genetic engineering technology is such that they can live much longer. It's not enough to stop The Fog of Ages setting in though, so their bodies are designed to shut down while their mental faculties are more or less intact.
 * An immortal from Ghost Sweeper Mikami who has forgotten the formula that made him immortal.
 * Invoked in some Bleach Fanon to explain why Shinigami generally don't remember their mortal lives- they measure their ages in hundreds.
 * The manga Phoenix features a historical warlord who seeks to claim the blood of the eponymous bird, and with it, immortality, mainly so his empire won't fall into the hands of his incompetent sons. He decides against this when he gets a glimpse of himself in the future, practically invalid and bound to a machine that erases his memories so he has enough brain power to function.
 * C.C. from Code Geass suffers from this,
 * In the Samurai Deeper Kyo manga, this is the Start of Darkness of Magnificent Bastard Chinmei.

Comic Books

 * An issue that explores all the back stories of the minor characters in Invincible reveals that The Immortal has forgotten some of his past experiences. Including being Abraham Lincoln.
 * Also in an alternate future Invincible had been taken to where the Immortal took over the world, he barely recognized Invincible and didn't remember his own wife.
 * He had trouble recognizing Invincible because Invincible was still alive in that future, but had changed over the years. And judging from his reaction to the mention of his wife, he may not have forgotten so much as repressed the memory of her.
 * Often (but not always) averted by Vandal Savage, one of several immortals in the DC Universe. However, it has been hinted at or implied that Savage may also have some other powers.
 * Played straight in an early post-crisis story where he laments how much advanced medical knowledge (from forgotten civilizations he used to rule) he has lost over the ages. When a modern geneticist hesitates in assisting him with some human testing, he bellows that he has 'forgotten more than you'll ever learn!'
 * Det. Christian Walker, the main character of Powers, is actually an immortal who's been around since caveman days. He just doesn't remember anything before the early 20th century. His Arch Nemesis/Evil Counterpart who has been around just as long, on the other hand, seems to remember most of it. However, at the final tragic confrontation between the two, Walker demands to know why the nemesis has done the things he's done, and why the two have been fighting all this time. His enemy pauses, then admits that he can't remember anymore.
 * Wolverine, although some of this is due to Laser-Guided Amnesia; other explanations have been simply that he can't remember more than a lifetime of stuff he's done, or that it's an unfortunate side effect of his Healing Factor.

Film

 * Brought up in The Man From Earth - John only remembers "the ups and downs" and not all that much more outside of general details.
 * Could be it's not really this trope, as the "ups and downs" are generally what everyone remembers, ancient or not.
 * Max Schreck, the vampire actor in Shadow of the Vampire, suffers from this; most of the memories of his early life and his sire have faded, and throughout the film he claims to have forgotten killing members of the film crew less than a few hours after doing so. However, it's implied that Schreck isn't a "complete" vampire, given that he has continued aging despite being immortal, and that he was never capable of siring vampires of his own.

Literature

 * Professor Urban Chronotis from Dirk Gently's Holistic Detective Agency by Douglas Adams is an immortal time traveler who is explicitly capable of remembering about as much as a normal human. He doesn't forget things in chronological order, though; he just gets very absent-minded.
 * In particular, he believes he will always remember Cleopatra.
 * In particular, her ears (attractive ones, with big dangly blue earrings) and smell (from living in ancient Egypt, and possibly bathing in asses' milk).
 * The Nonmen of Bakker's Second Apocalypse series are afflicted with a particularly nasty version of this, in which
 * In David and Leigh Eddings' Belgariad, Belgarath the Sorcerer has lived for 7000 years. He can remember sensations of his mother, but not her face. Nor does he remember exactly which God's peoples he belonged to.
 * He does state, though, that he's not sure if this is because of how very old he is or because he just didn't consider it important enough to really notice at the time; he does mention that until he met Aldur properly, he didn't really consider religion to be important. Plus, his mother died when he was very young.
 * The Madness Season by C.S. Friedman has an energy based species of creatures which are virtually immortal with this problem, and which therefore prefer to live in symbiotic relationship with physically bound creatures.
 * Robert A. Heinlein's Lazarus Long comments in Time Enough for Love after over two millennia of existence: "I told you my memory was playing tricks. I've used Andy Libby's hypno-encyclopedic techniques - and they're good - and also learned tier storage for memory I didn't need every day, with keying words to let a tier cascade when I did need it, like a computer, and I have had my brain washed of useless memories several times in order to clear those file drawers for new data - and still it's no good. Half the time I can't remember where I put the book I was reading the night before, then waste a morning looking for it - before I remember that that book was one I was reading a century ago."
 * In the Harry Potter books, very old wizards like Dumbledore avoid this by storing important memories in an enchanted chalice called a Pensieve. Which is useful in other ways as well, such as making it possible to pass those memories on to other people as needed.
 * Crystal Singers in Anne McCaffrey's series of that name have this problem, though it's brought on more by long-term exposure to Ballybran crystal than actual age.
 * Age appears infinitely extendable in Terry Pratchett's Strata, and "memory surgery" prevents brain overload.
 * In the Discworld book Pyramids, the high priest Dios prevented himself from dying by reversing time by sleeping in a pyramid, but mentions that the process doesn't preserve memory. Instead, he refers to the written history of the the kingdom as his memory. As a result, he can't escape a millenia-long Stable Time Loop. By the time it comes around again it's a surprise.
 * In Kim Stanley Robinson's Red Mars Trilogy, advances in medicine let people live to over 150, but their memories start showing significant deterioration.
 * This is pretty much cured by a drug cocktail that apparently 'refreshes' the taker's memory, to the point where they have highly detailed recall of practically their entire lives.
 * In Robert J. Salyer's novel Starplex, the protagonist meets, who has forgotten.
 * The Minotaur is still alive today in Steven Sherrill's The Minotaur Takes A Cigarette Break, five thousand years later. His days in the Labyrinth are very vague to him.
 * A point of Glasshouse by Charles Stross. Immortality means that humans need to periodically erase their memories to make things more interesting. The protagonist has just done this when the book begins.
 * The Struldbruggs in Jonathan Swift's Gulliver's Travels have the opposite problem—once they turn 100, they get a case of short-term memory loss so bad they can't even remember how the sentence they just finished began. (They don't die of old age, but they do keep getting older, so they become intensely senile.)
 * Explicitly averted in Stanislaw Lem's Observation on the Spot where ectocs have a photographic memory and can easily remember anything that happened centuries ago, with them being made of Nanomachines. But that's just another reason why there's just six of them left.
 * In Stephen Baxter's Exultant, Luru Parz is one of a group of immortals who have survived more than 20,000 years. She claims that they can remember events from throughout their lives, but no more or less clearly than a normal person. Sometimes, events may bring forth a distinct memory that hadn't been recalled in several thousand years. Even so, they must "edit" their memories, but it isn't explained how this is achieved.
 * A Fire Upon the Deep. Peregrine Wrickwrackrum, a storyteller among the Tines (a pack of dog-like aliens who form a group mind) claims to have memories from his ancestors going back to the beginning of time, but admits that after you go beyond a few hundred years you can't tell the difference between legend and memory.
 * In Poul Anderson's World Without Stars, Humanity has cracked the immortality problem, so that no one dies of old age anymore. Every century or so people have to have old, unwanted, memories wiped, in order to make room for new memories.
 * Many of the beings from the far future in the novella Starplex suffer from this. A species known as the Ibs suffers from a related problem—their natural cause of death by old age is that their memories began to overwrite their autonomic routines. In this they are unlike most other sapients, who have problems with their cellular structure such as telomere shortening; the Ibs are noncellular.
 * The eponymous character of The Vampire Tapestry loses his memory each time he passes into hibernation, and speculates that this is a defense-mechanism against this trope.
 * Oddly, this trope seems to apply to Hazel-rah in the epilogue to Watership Down, in which the venerable Chief Rabbit can't recall if the adventures attributed to him in his youth were real or not. On the one hand, said adventures couldn't have happened so long ago by human standards; on the other, Hazel is implied to have vastly outlived what's normal for wild rabbits, suggesting that his lapine brain's memory capacity has indeed reached its limit.
 * Lampshaded in "Letter To a Phoenix", a short story by Fredric Brown which is told by a narrator who is 180,000 years old (he ages one day per 45 years). He states he doesn't remember his own name because he only has enough place in his head for the important facts - and what could be less important than a 180,000 year old name he changed about a thousand times already?
 * In Alastair Reynolds's House of Suns, the long-lived protagonists who've lived through six million years (though, admittedly, only a couple tens of thousands of those concious) routinely re-arrange their memory. It's implied that they could hold all of their memories at once, if they wanted to, but having that many memories would affect your personality so drastically that most choose not to. Most the long-lived characters tend to hold a rough cliff-notes version of their memories in their heads, but not any of the details; tne main character intentionally prioritizes "recent" memories, which in part drives the main plot.
 * Also by Alastair Reynolds, Zima Blue focuses in part on Arthur Zima's quest to find out where his obsession with a particular shade of aquamarine comes from.
 * Gilgamesh - yes, that Gilgamesh - from The Secrets of the Immortal Nicholas Flamel has this problem. He is the oldest immortal, the only being that is truly immortal, (Elders and other immortals can be killed in battle) and has gone insane because of it. He has tried to kill himself a few times; one attempt involved standing under the test of the first atomic bomb.
 * Flamel himself suffers from this sometimes. He once forgot how to speak English.
 * Khayman in Queen of the Damned suffers from this. As the third vampire ever in existence, he has spent the last 6000 years continuously active, alternatively losing and regaining his mind over the centuries, thus remembers little of his own life. It's been implied that during his periods of sanity, he engages in a game of manhunt with the Talamasca, who study the supernatural, while simultaneously writing many treaties on the origins of the vampire race as a member simply because he likes to mess with them.
 * Vampires in Twilight have Photographic Memory, but Alice cannot remember anything from back when she was human.

Live Action TV
"Doctor: Oh! Look at me, I'm thick! Old and thick! Head's too full of stuff, I need a bigger head!"
 * In the last episode of Angel, the eponymous character asked Harmony if she remembered what it was like being human because he no longer remembers himself.
 * Earlier Darla, resurrected human, realized she no longer remembered what her name had been before she became a vampire.
 * Methos from Highlander the Series. When he tells MacLeod that he's over 5000 years old, he explains that's when he took his first head and "before that, it all starts to blur." Methos therefore has no idea how much over 5000 years old he is; for all he knows he could've gone for thousands of years before meeting another Immortal for the first time and killing him.
 * The X-Files ("Tithonus"). A man who couldn't die because he'd "missed his chance" tells Scully he went to the records office once to look up some facts on his late wife, only he couldn't remember her name.
 * Death from Supernatural can't remember if he is as old, or older than, God. God, apparently, doesn't know either.
 * Doctor Who. Word of God has it that the Doctor can't remember how old he is anymore, instead going with the generic 900 years whenever the issue comes up.
 * In the episode World War III it's a plot point that his memory is so crowded and cluttered he needs help to remember the species the Slitheen come from, and their Weaksauce Weakness.
 * Heck, he does that a lot of times. It can't help that he also remembers multiple timelines.
 * Also explicitly mentioned in Silence in the Library when it takes the Doctor a while to recognize the Vashta Narada.

""You are enormous parts of my life. And you are all I ever remember.""
 * At least he remembers the important things. In the short Good Night he has this to say when Amy comments that his companions are such tiny parts of his life that he must hardly notice them:


 * Earlier episodes from the original series back this up. When in regeneration psychosis, The Doctor often mentions previous companions, either for no reason, or referring to his current companions by some other name.
 * According to Seven of Nine from Star Trek: Voyager, the Borg also suffer from this, as their memory from over 700 years ago is beginning to fragment.
 * An episode of Curiosity featuring Adam Savage was depicted as a future autobiography he wrote at age 1000. Among the other technological interventions he'd resorted to, to prolong his lifespan, was linking his brain to a computer so he could use its memory banks to supplement his own overfilled storage capacity.
 * Parodied on Canadian teen sitcom Mr. Young with Mrs. Byrne, who has a memory span of a few seconds due to having lived through an ice age.
 * According to the fan favourite episode "Brigadoom", the Brunnen-G of Lexx suffered pretty heavily from this trope, after retreating behind a nigh-impregnable shield on Brunnis II and cracking the immortality problem. It's implied this contributed to their insular behaviour and eventual downfall.

Tabletop Games

 * Vampire: The Requiem is the Trope Namer. One side effect of torpor (the comatose state vampires experience when they run out of blood or are beaten into unconsciousness) is that, the longer the vampire sleeps, the more their memories shift. A two week nap is no big problem, but if you're asleep for decades... well, you ever try remembering every detail of a dream? Yeah. It's like that. And as the blood "thickens" as a vampire gets older and they have to resort to even more potent sources of blood (animals, humans, eventually other, more powerful vampires) to stay alive, sooner or later, every vampire goes into torpor.
 * There is a bloodline known as the Agonistes, introduced in Bloodlines: The Chosen, who have devoted themselves to subverting the trope. Elders hire them when they prepare for torpor, and the Agonistes first record everything they can before using their special devotions to drive out as much of the fog as they can. They are very good at their jobs... and have received no end of persecution, as many vampires would prefer certain facts to be lost to the ages.
 * The Usiri of Ancient Bloodlines can also protect the memories of torpid vampires. Unfortunately, their true power is in pumping the spirits of torpid vampires for anything they can learn.
 * Naturally, some vampires may have learned to exploit this to their advantage. For example, an elder about to go into torpor could write down exaggerated or even fabricated stories of his own exploits and have them filed away in some dusty library, only to be dug up centuries later when he awakens. Now he has "genuine" documentation to support whatever claims he would like to make.
 * Occasionally, this backfires. One bloodline from Ancient Egypt, the Bak-Ra, believes they were once able to walk in the sun. In truth, it was an intensely powerful magical illusion, but good luck trying to convince a Bak-Ra awakened in the modern era of that.
 * The base book also goes into another reason why not every vampire writes things down to avoid this - due to the paranoia that pops up after dealing with other predatory monsters for countless years, they're not always certain that their diaries, records, and the like will be unaltered when they do wake up. They could try writing in code, but there's also no guarantee they'll remember the code, either.
 * Immortal Sinners, the Night Horrors book for Requiem, also shows the rare subversions-certain very old, very powerful vampires, called Methuselahs, have learned to cheat the Fog of Ages, meaning they remember everything they care to. The example still makes grandiose claims about who he is, but that's because he's a habitual liar and Trickster Archetype for mainly does things For the Lulz (and his real origin story is still pretty unbelievable).
 * In a Warhammer 40k background story, this is shown to be a problem for some Chaos Marines (the oldest of them being over 10,000 years old). The marine uses a special mental ritual to sort through and 'store' any memories from the past year he wants to keep (it turns out the only thing he feels worth keeping is killing a Space Marine).
 * The 'World on Fire' campaign setting from Spycraft has the Immortals as one faction. They succumb to this—at least, the ones who don't die from 'live fast, die old'.
 * In Classic Dungeons and Dragons, the oldest of Immortals don't recall having ever lived as mortal beings. It's implied that they simply can't remember their mortal lives; Korotiku, for example, speculates that he might have been a planar spider. Note that one of the Immortals who recalls his mortality quite clearly happens to have begun his life as a dinosaur, so the ones who've forgotten must be considerably older than that.
 * Kaa may be a justified exception: his focus is the preservation of cultures, knowledge, etc., that would otherwise be destroyed.
 * The Sindar (elves) of Hârn suffer from an extreme form of this, to the point where they will completely forget friends after a long absence. Often an elf will remember songs and tales of events he took part in but have no memory of the actual events.

Web Comics

 * Immortals in El Goonish Shive have this as inherent to their nature, with a slight variation. Every couple of centuries, they "die" and lose the vast majority of their memory and power. They can apparently choose the time of their death, or even postpone it indefinitely, but this has exactly the result you'd expect.
 * It's actually said to be a willful decision. Living forever without any kind of reset, besides the possibility of going crazy, can be just plain old boring. As such, it's less something that happens that they can postpone, and more something they simply decide to do to avoid madness and soul-crushing boredom. They do retain their memories, but memories from past 'lives' are described as though they were being read out of a book, and as a result, probably suffer from the Fog of Ages.
 * According to the background info for DMFA, the oldest living demon is tens of thousands of years old (a normal demon life span is 1500 years) but a side-effect of whatever forgotten process that granted her such longevity is that she cannot remember back further than 100 years before the current day.
 * The Branthicor in Schlock Mercenary intentionally designed their own brains to work this way. Their normal brains couldn't handle becoming functionally immortal due to problems with long-term/short-term memory storage, but their new brains can... If only by intentionally 'compressing' long-term information to the point where they suffer from this trope. It beats the alternative.
 * Jin of Wapsi Square has lived for over 80,000 years due to immortality and a Groundhog Day Loop, and, as a result, she often has difficulties remembering details from previous cycles. She often entrusts people with information to help her remember.

Video Games

 * The earliest known example of this in a video game is Glory of Heracles III, for the Super Famicom, where the protagonist of the game is an immortal who suffers from amnesia. This plot element is used again in one of the sequels, Glory of Heracles for the Nintendo DS, where the protagonist is also an amnesiac immortal.
 * Ellen from Touhou Project is an immortal witch who loses her memories blocks at a time, and so she acts as childish as she looks.
 * Fairies in general, and Cirno specifically, fall into this category - as Anthropomorphic Personifications of nature, they are immortal to From a Single Cell levels for as long as their aspect of nature is powerful enough to support them. Individual fairies may well be Time Abyss material, but because they are also permanently childish and simple-minded, they often forget everything that happened yesterday, much less a thousand years ago. Cirno, for example, can only vaguely remember the previous occurrence of the flower's odd growth from Phantasmagoria Of Flower View, only after being reminded a few times, and can't remember any of the details, but is fine with it, since she doesn't care, anyway. She simply wants to do what she always does - play, fly around, and pick fights to see if she'll win.
 * Fujiwara no Mokou, an immortal, somewhat played this trope straight. In a supplementary material for a manga, it's explained that her rivalry with another immortal that's often thought to have driven her to take the immortality elixir wasn't - she had forgotten about her by the time she was tasked to dispose of the elixir and was more interested in the prospect of, er, being immortal. Her rivalry is more out of a sense that she has something constant in her life, now.
 * Used in part in Lost Odyssey. The main character is an amnesiac immortal who's lived for a thousand years; most of his memories are recalled through dreams as the game progresses. It's not a natural side effect of the immortality, though, but rather a case of Laser-Guided Amnesia that makes him more easy to manipulate.
 * Another character in Lost Odyssey does in fact keep journals; when that character experiences amnesia, the journals prove to be very helpful.
 * In Planescape: Torment, this is implied to have happened to Ravel Puzzlewell—she's certainly not 'all there' when you meet her. As for The Nameless One himself, he is immune to this trope: He does suffer heavily from memory loss, but it's caused by him repeatedly dying and coming back instead of from living too long.
 * According to Chris Avellone, Ravel's apparent senility actually stems from the fact that due to her nature and having all of her "branches" (including Mebbeth, Marta and Ei-Veine), she sees across many planes and time periods all at once and she sometimes has trouble distinguishing exactly where or when she is (hence some of the strange diaglog—it was meant to be spoken by another one of Ravel's "selves")
 * In 7th Saga, the robot Lux was built with a finite amount of memory and as a result has forgotten who built him and why.

Web Comics
"Petey: I might - MIGHT - be immortal, but on galactic time I'm still a baby. Look around! If it's this easy to live forever, where are all the grown-ups?"
 * The Bradicor in Schlock Mercenary developed physical immortality, but ran into some nasty problems.
 * Even the local computer "god" is concerned.

Web Original

 * In Orion's Arm this can easily happen to nearbaselines who live more than a few hundred years. Though cybernetic and nanotech enhancements to memory are usually widely available, it is still rare to live longer than about 1,500 years without transcending.
 * Three (aka Clarion, Silver, Assistant...) has been jumping from universe to universe for about 30-50,000 years, give or take. She can't remember exactly how old she is. She's also forgotten what her species or homeworld was called and possibly even her original name. She does try to hold on to certain memories, particularly those of close friends, lovers, and adopted children.

Western Animation

 * Paradox from Ben 10 Alien Force is so old, he can't remember his own name, hence why he adopts the moniker.
 * He also had a small bout of insanity, which couldn't have helped.
 * The fact that he exists outside of linear time probably doesn't help either.