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.
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 Somnus 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.
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In Slayers, the elves created a powerful weapon that is immune to all magical attacks. It could also spit out smaller duplicates. Unfortunately since the artifact is immune to all magic, it cannot be controlled. So, the elves just buried it and hope no one will ever find it.
Doomsday itself. He is created on prehistoric Krypton with the sole purpose to destroy all life with an absurd amount of brute strength to back it up.
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.
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.
In the Stargate setting, the Ancients seemed to make a hobby of building these : most of the plots of Stargate Atlantis that aren't related to Wraith come from ridiculously dangerous unlabeled Ancient stuff. Downplayed, in that most of them had an actual purpose (killing Wraith), they just forgot to add, well, any failsafe or documentation. 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 and re-seed the galaxy with life.
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.
Well, they actually did do a pretty thorough job, but since the Replicators are called that way for a reason, the few who survived rebuilt the civilization. So it's kind of like throwing an Ebola-virus sample away and assuming nothing wrong will happen. To their credit, the war with the Wraith is implied to have been quite advanced by that point, so maybe they just didn't have the resources available for a proper cleaning.
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, plus exponentials amounts of energy (and, of course, the device can't be turned off once it starts to overcharge). 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 built it in their own city), 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.
A weapon used for destroying Stargates. It can be used to maintain a wormhole indefinitely (Beyond the 38 minute limit), but doing so causes the gate on the other side to eventually overload and explode, taking most of the planet with it.
Not to mention, you know, Atlantis. Which given the chair platform, super shields, and the Ancient database, probably qualifies as a whole squadron of doomsday devices, at the very least.
A God Game. Which worked by orbiting a few hundreds satellites over some inhabited planets, and a console to give the locals orders. Built by the same guys who, one ascended, forbid any interfering with lower planes because "Playing god in wrong".
Noticeable in that Mc Kay and Sheppard had it mistaken for an Ancient Video Game. Which means that either they built it without considering it any different from their ordinary video games, or their only one video game is about tricking real humans into thinking you're a god.
A machine altering someone's DNA to make them ascend. Or die trying.
It says something on the Ancient when the only non-ascended named one of the entire series, Janus, was kind of a Mad Scientist who built quite a few of these.
Like a working Time Machine. This one is an inversion, though : it's actually useful, doesn't have any nasty side effect we know about, and has a great potential for, you know, world saving and the like. Still, the Ancients were frightened of it and ordered Janus to destroy it (he didn't).
The Attero 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. To his credit, the labo where Janus had left it was hidden fairly well, and he put a note on it which indicated "Has nasty side effects", making the Attero Device the best documented Ancient device ever.
The worst thing about it is that, due to the aforementioned time machine, the Ancients were warned that Earth humans would one day visit Atlantis and the Pegasus Galaxy. They prepared accordingly, by programming a failsafe on Atlantis "submarine mode" (it resurfaces when the batteries that power it's shield are empty), an hologram about the Wraith controlling the galaxy, and lights that turn on when you arrive, which was nice of them. It would have been nicer to, say take a few hours a destroy or at least document their explosive-tumors giving machines, leave a side-note about the Replicators, and a list of planets not to go to (because, you know, the Wraith control them) before they were ready. But, since most of the series' plots wouldn't have happened then...
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. The one Kirk destroyed was also just the prototype. A far more powerful one has to be dealt with over the course of the novel. Ultimately very very pointless as it attempts to go to warp 10 and gets stuck in the Dichotomy motion Paradox. Amazingly, references to transwarp (Voyager) don't really break continuity as you need special gizmos go into transwarp. The ship was just using a regular warp drive. This Troper assumes the Warp 10 limit didn't exist when the novel was written, but it was a TNG novel so who knows.
Larry Niven's Known Space novels and short stories have one of these in the distant background, a device which supposedly ended the "Slaver Wars" and left fragmentary populations of survivors to survive into the distant future.
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.
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, the Fifth Empire is already well on its way to restoring its military might to the Achuultani-raping levels of the Fourth (with centuries to spare), 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 a rapidly-evolving super plague that destroys all life, has a very long dormancy period, and cannot be cured. That it turns out they could have effortlessly wiped out the invaders with their current military power just makes things worse.
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.
The Magician's Nephew has the Deplorable Word which, once spoken, will eliminate all life in the universe other than the speaker. Its pointlessness is sort of the point.
Played with in Warhammer 40,000. Finding ancient doomsday weapons with no clear purpose evident is pretty common, but the setting being what it is, pretty much all of them did have a purpose at one point in time. The Blackstone Fortresses, for example, seemed pointless until the Necrons, whom the Fortresses were built to combat, returned.
Many First Age superweapons in the Exalted setting fall in that category. After all, we are speaking of a Golden Age where the war against evil was supposedly won, so why the hell did the Exalted continue to manufacture and stockpile world-theatening weapons in such extents? Sure, the defeated enemies (Yozis, Neverborns and Rashka) are still around and that totally justifies some things. The Realm Defense Grid, for example, is a rather well designed superwapon, which has world-shattering capabilities but can also be used for more small-scale and limited warfare, and comes with serious security systems and safety mechanisms. But honestly, what good can come from things like Thousand-Forged Dragons, Soulbreaker Orbs, or mass destruction Solar circle spells such as Rain of Doom? They are clearly more suited to devastating Creation and killing a massive number of people than to waging an efficient war against demons, undead or fairies.
The only people responsible enough to make something about that after the Primordial War are the Maidens of Destiny, who sealed away the most powerful weapons of their servants (the Greater Signs and the Astrological Charms, which basically allow you to directly rewrite the fabric of the universe) and hope they will never need to be used again (although Saturn is probably no fool about this — after all, there is always an ending...)
Forgotten Realms — The Dracorage Mythal, a device designed to put every dragon in the world at once into murderous madness. As an added bonus, the damn thing actives automatically every time a specific comet passes overhead. It had a logical purpose at the time of its construction and initial activation, but by the time it had thoroughly served that purpose (smashing draconic civilization beyond all hope of reconstruction) no-one alive knew how the Dracorage Mythal worked, or where the control system that could be used to shut it down was.
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. The fact that it ultimately lead to the firey downfall of the settlement is more a tribute to its epic mismanagement.
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 WEAPON 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?
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 at first, after reclaiming the fortress they rebuilt FAILCANNON and are currently testing it as we type this
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 Mycon and Daktaklakpak).
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 VI, 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 — but proved mostly useless in saving it too.
Admittedly, one time it was used, there was a meteor strike immediately after. It is a little unclear about whether the Mana Cannon itself destroys civilization or if the users just have really bad timing.
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.
Final Fantasy X-2 has Vegnagun. It is a powerful superweapon, yes, and would probably be actually quite useful — if it didn't destroy the entire world upon firing.
If fired up at the roof of an underground cavern, then yes it will blow the surface above to smithereens. Presumably that is not how it was meant to be used. The reason it's sealed away is because it has a broken IFF.