Pointless Doomsday Device

"Self destruct lever. A mechanism that, for example, could flood your fort with magma, or release a trapped megabeast. For bonus points, build the whole fort on a single support. Difficulty: Very high. Extremely dangerous. Usefulness: None, by definition, but highly amusing."

- Dwarf Fortress Wiki

The Time of Myths was a dangerous age to live in, with Eldritch Abominations on the loose, evil gods eating souls and whatnot. So it's not surprising that after becoming Sufficiently Advanced, the Precursors made superweapons to defend themselves and make the galaxy or planet a safer place for their descendants. Conscientious precursors will dismantle or at least disable these weapons while preserving them for later use—usually by scattering the component Mac Guffins across the cosmos or placing some kind of condition for activation (say Virgin Power, or innocence).

Of course, most precursors tend to be either neglectful or outright abusive. What this means is that they will make these superweapons for no particular reason other than that they can. And even Benevolent Precursors might have rogue elements or simply have their superweapons seemingly destroyed and thought lost amid the casualties but in fact thrown through time. The weapon itself may have had legitimate use once upon a time, but after making the planet destroying espresso machine to defeat Insomnus the Snorer of Worlds, they will completely forget about ever having made it and leave it in an easily accessible location. The device will (of course) be immune to the ravages of time and ready to use minutes after being found. Worse, the precursors may not have even labeled the thing! So for all intents and purposes it seems like a normal espresso machine... which will destroy whatever planet it's on after one use. Of course, every villain or misguided soul in the setting will do whatever it takes to possess and activate the device, sometimes without even realizing (or underestimating) the danger it poses.

Alternatively, it represents something powerful with great potential for abuse that is knowingly left behind. Lastly, it can also include examples of Sealed Evil in a Can where the evil could easily have been defeated, but wasn't for no good reason, forcing people to deal with an unrepentant and now further insane monster in the future.

It's as if the the US and Russia decided to "get rid" of all their nukes by putting rune encrusted marble doors outside every missile silo and left it at that. Oh, and the runes spell out "Come in, there's punch and cake! Just turn both keys at the same time".

Not to be confused with the "plain" Doomsday Device.

Anime & Manga

 * In Slayers, the elves created a powerful weapon that is immune to all magical attacks. It could also spit out smaller duplicates. Unfortunately

Comic Books

 * The Ultimate Nullifier is a device that can disintegrate anything (and sometimes more than that, depending on the continuity)--up to and including the entire universe. It's been used to stand off Galactus, but it's not at all clear why the thing exists in the first place, or why it's kept in Galactus' starbase.

Film
"Crow: Let's go drop it on some other planet, see who bites."
 * In the So Bad It's Good movie Puma Man, the artifact in question is a mask that can control minds. This mask was designed by a race of Sufficiently Advanced Aliens who believe that "Each man is a god, each man is free." This mask has no purpose other than to steal people's will, it can even be used to force people to commit suicide.


 * Plan 9 from Outer Space: The aliens' motivation for (ineffectively) terrorising Earth is because they believe if humans continue to advance, we'll create exploding sunlight that will cascade-detonate the universe.
 * The Russians' Doomsday Machine from Dr. Strangelove actually had a very good point to it: to prevent nuclear holocaust by destroying the entire world in the event of a nuclear exchange - the ultimate form of mutually assured destruction. Unfortunately, the Russians hadn't gotten around to telling anybody about it just yet, since they were saving the announcement for a special occasion, because, as the ambassador puts it, "The Premier loves surprises". As the titular doctor points out, a deterrent isn't very deterring if nobody knows about it, which turns it into this trope.
 * The Alpha-Omega Bomb from Beneath the Planet of the Apes, capable of incinerating Earth's atmosphere, was built only as a deterrent. Too bad a millennium later it would be the god of a mutant group...

Live Action Television

 * Buffy the Vampire Slayer has multiple doomsday devices, spells and other various things.
 * In the Stargate setting, the Ancients seemed to make a hobby of building these. Prominent examples include:
 * the Dakara Superweapon: capable of selectively disintegrating throughout the entire galaxy a particular species of life or all life in general, depending on how you set its incomprehensible but easily-used controls. Originally built to stop a plague.
 * the San Graal: capable of destroying all Ascended beings in a galaxy - granted, this one was actually intended to be a weapon, and deliberately hidden behind puzzles.
 * Actually, it was instructions to MAKE the San Graal, and the good guys were given some assistance to find it, so this may be more of a conscientous precursor than Pointless Doomsday Device.
 * the Asuran Replicators: a form of machine life built as a weapon to destroy the Wraith but then abandoned to its own devices when it proved to be a "failure" in some undefined way. The Ancients at least did try to destroy it, but of course didn't do a particularly thorough job.
 * Actually they did do a pretty thorough job, unfortunately out of the uncountable trillions of nanobots that make the Asuran Replicators they missed a couple, which was all that was needed to rebuild themselves.
 * Project Arcturus, an attempt to build a super-powerful power source that turned out to be highly unstable and that emitted unstoppable deadly radiation when in operation. Killed everyone on the planet with its radiation, but they left it in place for future tinkerers to mess with (and eventually wound up exploding, destroying 5/6 of the solar system it was in)
 * After the original blew up, the Atlantis crew revisited the concept, and this time almost managed to destroy a parallel universe. Apparently they were fast learners.
 * The device that gives anyone exposed to it exploding tumors. If such a thing existed.
 * The Ancients were losing the war against the Wraiths and were desperate for any weapon that could turn the tide. However, as the Wraith attacked Ancient worlds, the Ancients had to abandon most of the research and prototypes. Since they were convinced that they would eventually win and reclaim all that they lost, they did not bother destroying or neutralizing all those DoomsdayDevices. During the final evacuation from Atlantis they even left a functioning time travel device behind.
 * A weapon used for destroying Stargates. There is no other conceivable use for the device, since it is stationary, and the beam it projects is specifically tuned to detonate the gate on the other end of the wormhole. Bear in mind that Stargates targeted with this weapon explode with enough force to destroy an entire planet. Truly, the Ancients were paragons of morality and pacifism.
 * The Altero device, which was specifically tuned to Wraith hyperdrives to cause them to blow up during a jump throughout the galaxy. Unfortunately, it had the nasty side effect of causing every active stargate to explode in a planet-busting manner. Whoops! To their credit, they have actually hidden the lab fairly well.
 * Once the Ancients ascended they stopped caring about all the deadly technology they left behind and regarded any attempts to fix this as forbidden interference with non-ascended beings.
 * The planet-eating "doomsday machine" that appears in an episode of Star Trek: The Original Series seems to be one of these. Kirk can only guess as to where it may have come from or why it was activated, but he still has to stop it before it destroys anything else.
 * His guess is that it was built for an ancient war that neither side survived after it was activated. If true the trope wouldn't apply, since it wasn't forgotten so much as it just kind of wandered off while still in use.
 * Peter David's TNG novel Vendetta expands on the doomsday machine: It was built as a last-ditch weapon by a race which had been nearly wiped out by The Borg. The one that Kirk destroyed was on an automated course, not to Earth, but through Earth to get to Borg territory.

Literature

 * Larry Niven's Known Space novels and short stories have one of these in the distant background, a device which ended the "Slaver Wars" by destroying everything with a spinal cord; leaving only fragments of the Kzin civilization behind.
 * The Star Wars Expanded Universe features the Shawken Device, which is supposed to destroy the universe. It is notable for operating on principles that are considered dubious at best even by characters in the comic book in which it appears. But nobody really wants to test it, just in case. Luke and his sort-of apprentice even go to great lengths to disable it.
 * Also from the Star Wars EU is Centerpoint Station, a massive space station with the power to both move and destroy celestial bodies. Built by a ancient race called the Celestials, it doesn't seem to serve a purpose besides being a handy superweapon for the plot.
 * The Hypernova Bomb created by Hactar, and "given" to the Elders of Krikkit, in Life The Universe And Everything.
 * Hactar was original designed and built by the Silastic Armorfiends of Striterax and tasked with creating them an Ultimate Weapon. On asking what they meant, it was instructed to "read a bloody dictionary". Whilst indeed pointless (it would blow up the entire Universe, leaving no victors but indeed making it the last weapon ever) it was a nice example of how the customer may be stupid but they are never wrong.
 * In the third Empire From the Ashes book, the Fourth Empire's plans for an extremely-advanced gravitronic bomb capable of destroying a sun—all on its own—are discovered. By this time,, so there's really nothing else to do with the plans besides let them fall into the hands of a highly-organized, widespread group of religious terrorists bent on toppling the government for allying with the minions of the Anti Christ!
 * The "bio-weapon" of the Fourth Empire probably qualifies for when it was invented, as well. No amount of paranoia over the yet-unseen wave of genocidal invaders can make other than mindbogglingly insane the creation of.
 * Possibly the Time Matrix from Animorphs. In The Andalite Chronicles, it was implied to have been created by the Ellimist(s), the theory being that he/they used it to Ascend to a Higher Plane of Existence in the first place. Years and dozens of books later The Ellimist Chronicles told the real story of how this happened, and the Time Matrix didn't even get a mention. So, who knows?
 * There is a bit of justification for it just being left behind, though—neither the Ellimist nor Crayak want the other's side to have it, so their "game" presumably has a rule about neither messing with it (until they had to in Elfangor's Secret). Still leaves its origin (and how it got on Earth before the Skrit Na found it) a mystery.

Tabletop Games

 * One of these left behind by the Blackmoor civilization, then found and tinkered with by some ignorant elves, is the reason why one region of the Mystara game-setting is known as the Broken Lands.
 * Forgotten Realms has the Dracorage Mythal. Once upon a time, the Dragons ruled Toril, after mostly extrminating the previous dominating species. Elves created a device that wraps the whole world in a magical field that turns the dragons into stupid raging beasts every time a certain comet comes close. Upon the first activation Dragons began to tear apart their own servants and offspring, and soon there was nothing left of their civilization and elves ruled the world for next millenia... then they torn themselves apart without any external help and were mostly displaced by giants and dwarves, then by humans. And through all this time the forgotten spell field never failed to activate and inflict upon the world its brand of nightmare-from-the-sky every few dozen years as the "King-Killer Star" flies by. Oh, and later one insane lich managed to tinker with the Dracorage Mythal enough to get it activated indefinitely independently of the comet. Oops.
 * Which also led to creation of another doomsday device, since the Dragons tried to solve the problem in any way possible, so they designed an enormous magical weapon to shoot the comet itself, which in its test firing missed and hit the moon, which gave Toril a new inner sea and a cluster of asteroids next to the moon (in a Trojan orbit). Also, the inabitants of said moon for some inexplicable reason remain extremely paranoid about the inevitable conquest from Toril despite apparent lack of evidence to this end. The dragons involved were either lost in the next Rage or killed in unrelated incidents, so the device is forgotten.

Video Games

 * In Dwarf Fortress, making these is a typical dwarven hobby. They usually involve magma. Notable examples from succession games include Boatmurdered's Project: Fuck The World (which incidentally shortens to FTW), and Headshoots'
 * Boatmurdered's FTW is more an example of a standard Doomsday Device, in that it was used repeatedly and to great effect protecting the fortress from everything from goblin invasions to Bronze Colossi to unfortunate merchant caravans who arrived at just the wrong time.
 * Best of all, succession games incorporate the "Lost Technology" aspect of the trope - within real-time days or weeks, as one player constructs such a device and connects levers to it, but then doesn't label any of the levers so that a few players later, somebody inherits it and literally has a) no idea how to activate the device if they wanted to and b) no idea which lever does what. The results are pretty much inevitable, especially given that tantruming dwarves often throw levers without being ordered to.
 * Boatmurdered also featured a lever to flood the siege workshop. None of the players could remember why (the room had formerly contained a farm plot).
 * Headshoots had one hilarious example; a failsafe switch for  is right next to and the same colour as a device that releases all the cats from a cage.
 * So saving the world involves killing your framerate?
 * Who said anything about "saving?"
 * In Syrupleaf, the same lever somehow got connected to both the outer drawbridges and the irrigation floodgates, so raising the bridge to protect the trade depot would flood the farm plots.
 * It used to be (It still is to a lesser degree) that anything caught under a closing drawbridge would be utterly destroyed, removed from the game. This was refered to as the Dwarven Atom Smasher, and was useful for everything from destroying enemy seiges to getting rid of garbage.
 * Check out the community pages (The forums and the wiki) for plenty more examples. Most notably, the wiki offers a suggestion that players build a 'doomsday clock'; a mechanism that will, if a certain pressure plate is not triggered for X amount of time or some other condition is not met, destroy the fortress and render it permanently uninhabitable.
 * The succession game famous as Boatmurdered 2010 Battlefailed had FAILCANNON similar to FTW but was designed so that the faucet was a giant skull. It lived up to its name
 * The Atlamillia in Dark Cloud 2. Imbued with the power of the Sun, Earth, and Moon, these stones will automatically summon the Star of Destruction upon the world if they're ever brought together. Because, you know, absolute power corrupts absolutely yadda yadda yadda. So... why create them in the first place?
 * Seeing as one of them managed to save the world in the previous game, it doesn't seem all that senseless.
 * Just whom did the Precursors of Star Control have to deal with that warranted the fleet buster that was the Sa-Matra? And why would they leave it behind when they left the Galaxy?
 * Well, we know the planet-busting bombs that was used to destroy it were engineering tools ...
 * And it is arguable whether they left the Galaxy at all. The device may have been intended for use to fix "mistakes" (like ).
 * The Ultima series has the Armageddon spell, which will wipe out all life in the universe. (except Lord British, and sometimes the caster is immune too. Sometimes.) In Ultima 6, you're given the spell as a freebie by the Xorinite, hoping to entice you into taking advantage of their information-brokering services. They don't think the spell is very impressive since it only affects the current plane of existence.
 * Played with in Tales of Symphonia and Tales of Phantasia. The superweapon, the Mana Cannon, wasn't left behind- the plans were. Despite the weapon ultimately being the destruction of every society to use those plans, its one of the first things to be built when magitechnology rebuilds from the inevitable knocked-back-to-the-stone-age that always follows its construction. Except in Symphonia, where it didn't destroy the world â€” but proved mostly useless in saving it too.
 * This was Dr. Ivo "Eggman" Robotnik's M.O. from about 1998-2008 in Sonic the Hedgehog. He would look for some Sealed Evil in a Can, often a god or Eldritch Abomination of some sort. It would almost always run amok until Sonic and his pals invariably showed up. Lately, however, he seems to be creating original plans again.
 * In Fallout, The Cathedral's basement has a fully functional atom bomb that can only be used to destroy the Cathedral. Just in case The Master decides his Master Plan is flawed beyond salvation and just wants to end it all instead of finding a new plan, or telling the people working for him. Also very convenient if some aspiring person with technical knowhow happens to disagree with the Master Plan and makes an unannounced visit.
 * The Thermonuclear Detonation spell of the Quest for Glory series. It causes an explosion with the strength of a nuclear warhead to occur - centered on the hero. This causes an automatic game over, since it kills you and destroys the nation you are trying to save at the time.
 * Final Fantasy X-2 has

Web Comics

 * The dragons' iridium bomb in The Inexplicable Adventures of Bob.

Western Animation

 * In Futurama, Professor Farnsworth has a collection of doomsday weapons, just sitting in storage. Even after using one for good in "Time Keeps On Slippin'", he claims to still have enough left over to destroy the world.