Mermaid Saga

"Rule 20: Don't eat mermaid flesh. Period."

—Livin' and Lovin' In The Anime Universe: A Basic Guide.

It is said that eating the flesh of a mermaid can give one eternal life. Yuuta, a poor fisherman, didn't really believe the tales, but when the rest of the village tried some, he ate some as well. Less well-known is that mermaid's flesh is fatal to most, and almost all those who don't die are transformed into hideous mindless immortal beasts called Lost Souls. Within days, Yuuta was the sole survivor of his village. 500 years later, he is physically no older, but has grown weary of his lonely existence, and seeks the one creature that might be able to restore his mortality—a mermaid.

In the first of the linked stories, he meets a new immortal (the first he's met in his life) -- a beautiful 15-year-old named Mana, who's been raised in utter seclusion from childhood while being groomed for immortality. The two travel together, in search of a cure that may not even exist—after all, what else is there for them to do? It's not as if they don't have all the time in the world. The other tales are divided among Yuuta's reminiscences of his past adventures over the centuries, and their new adventures in their quest for mermaids.

The manga was the creation of Rumiko Takahashi, famous for creating Ranma ½ and Inuyasha, but is far darker in tone. No over-the-top comic antics, but the terrible things that happen when humans try and seek immortality. Two OVAs were made of individual stories back in the 80s -- Mermaid's Forest and Mermaid's Scar. The former name was used as the title for a recent^[when?] TV series which covered almost all the stories (with the exception of Mermaid's Gaze; the Mermaid's Scar episodes were not broadcast, and were on the video release only). Most of the stories took two episodes each.

It's important to note that these are Japanese mermaids, with fangs, fish-eyes and an appetite for human flesh, rather than the siren-like western version. The mermaids are also immortal and (in the manga version at least) can likewise only be killed by beheading or poison. Some can take on human form, but then slowly age, and they need to feed on an immortal woman to take on a new human form - that of the woman they feed on.

Tropes used in Mermaid Saga include:

Applied Phlebotinum: Mermaid's flesh, blood, ashes and liver are all used at some point. Also, a magical Buddhist ritual called hangon is used in one story.
And I Must Scream: While most Lost Souls are mindless beasts, at least one of them still had a conscious mind.
Blessed with Suck: Immortality is pretty horrible (not immune to pain, outliving everyone you know, having to move constantly), even without having to deal with Masato forcing you to be his replacement Mommy. Also, due to the reveal that the rate of conversion is different for everyone, there's no guarantee that an immortal won't become a Lost Soul at some point in the distant future.
Body Horror: The process by which a victim is transformed into the hideous, hulking abomination known as "Lost Soul." Sometimes the process is incomplete, leaving only part of the victim's body changed.
Canine Companion: Shiro to Towa. The reason they are so close is because Towa feels like Shiro is the only one who understands her, as they are both deformed due to the effects of the mermaid's blood.
Celibate Hero: Yuta has actively shied away from any romantic relations after his first marriage due to his immortality. Too bad that he also seems to be a bit of a...
Chick Magnet: Not played for any laughs. In fact the story behind Mermaid's Promise was that Nae was murdered by her jealous fiance when he realized that she was in love with Yuuta instead of him.
Creepy Doll: There appears to be a life-sized one of a girl Yuuta used to know. Turns out, it's the actual body of that girl. Which, thanks to the mermaid's flesh she used for poison on herself, hasn't rotted at all since the day she died.
- That's assuming she did die - there's a suggestion that she may have actually been paralyzed and fully conscious.
Darker and Edgier: Than most of Takahashi's other works, at least.
The Ditz: Mana displays evidence of this at times due to her extremely sheltered upbringing, not knowing what cats are, how to make tea or the first thing about romance—the last of which adds an interesting element to her relationship with Yuuta.
Enfant Terrible: Combined with Complete Monster.
Evil All Along: Sawa Kannagi and Masato.
Eye Remember: In Mermaid's Gaze.
Failure Is the Only Option: The first story claims that there may be no cure after all—but Yuuta doesn't see any better options than to keep searching.
Flying Dutchman: Yuuta can't stay in any one place for too long for fear of his eternal youth causing resentment.
Good Thing You Can Heal
Idiosyncratic Episode Naming: Most (though not all) of the episodes are named Mermaid's <noun>.
I'm a Humanitarian: Not only do mermaids and humans eat each other's flesh, but mermaids in human form need to feed on the flesh of their own kind when pregnant before they can give birth.
Locked Into Strangeness: Towa Kannagi.
Madwoman in the Attic: Towa's main grief with Sawa is that the latter locked her away to prevent other people from seeing her, while Sawa herself lived a full and happy life with a spouse and children.
Mayfly-December Romance: Yuuta's marriage 500 years ago became poisoned by his wife's resentment of his eternal youth. Since then he's refused to become seriously involved with mortals, although there have been several women interested in him.
Million-to-One Chance: Unlike most, the chances of becoming immortal from eating mermaid flesh seems to genuinely be one in a million.
Names to Know in Anime: Koichi Yamadera (Yuuta) and Minami Takayama (Mana). Also Makiko Ohmoto as Masato, and Rei Sakuma as his mom.
Nigh Invulnerability: Regeneration variety—all immortals and Lost Souls can be killed only by beheading, burning down to ash, or a poison made by rotting mermaid flesh in certain herbs. Everything else they just heal. Missing limbs and the like can also be replaced by grafting a replacement on, which leads to some particularly gruesome scenes...
Off with His Head: The traditional way of disposing of immortals and Lost Souls. Others get more creative.
Older Than They Look: Naturally. Special mention may go to Towa, who only looks young on the outside. Her internal organs are those of an old woman.
Our Mermaids Are Different: Very different.
Razor Floss: Shows up in Mermaid's Scar.
Really Seven Hundred Years Old / Older Than They Look: Probably about half the characters, thanks to the premise. For the main characters, Yuuta is the former (being about 500 but looking about eighteen), while Mana is the latter (physically 15, but probably around 30 by the end of the series.
Replacement Goldfish / Artificial Human: "The Bone Princess."
Self-Mutilation Demonstration
Skinship Grope: Takes on a chillingly sinister tone in Mermaid's Forest.
The Virus: "Lost Souls."
To Become Human: The main goal of the recurring characters.
Walking the Earth
White-Haired Pretty Girl: Towa
Whole-Episode Flashback: "The Village of Flying Fish" and "The Bone Princess."
Who Wants to Live Forever?: Played very straight.