These people are the reason why the precursors aren't around anymore. Either it is because their technology is so much more advanced than the precursors, or they were just so numerous that the precursors couldn't keep them at bay forever. Expect some sort of LostSuperweapon to be found that the precursors weren't able to finish in time to stop the Precursor Killers.

Often this is a SealedEvilInACan that was somehow stopped with the last actions of the precursors. Compare AbusivePrecursors and TurnedAgainstTheirMasters.

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!!Examples:

[[foldercontrol]]

[[folder: Fan Works ]]

* In ''Fanfic/WithStringsAttached'', the four are told that the invading Tayhil wiped out most Baravadans hundreds of years ago, an event that gave rise to the current skahs/tirin anarchistic civilization.
** In a twist on the theme, the last known Tayhil were killed only about ten years ago, and the skahs want them back because they're going crazy with boredom now that they have nothing to fight.
* In the ''Fanfic/PonyPOVSeries'', it's revealed that [[spoiler:Lord Tirek]] is the reason the [[{{Precursors}} Centaurs]] aren't around anymore.
[[/folder]]

[[folder: Film ]]

* ''Film/TransformersRevengeOfTheFallen'' has The Fallen as one of the first Transformers, the Primes. He killed his brothers in a mad quest for power, but they managed to hide the artifact he was looking for and seal The Fallen away for a time as they all died. As it turned out one Prime escaped his view, Optimus.

[[/folder]]

[[folder: Literature ]]

* The entire tragic backstory of the elves in the ''{{Literature/Deverry}}'' series is originally based on a bunch of dimensionally travelling gauls entering a different world and setting off a chain of displacements the end results which brought down the elven civilization.
* In the ''Priscilla Hutchins'' series of Creator/JackMcDevitt's novels, every major nonhuman society has disappeared but for a smattering of artifacts left around the galaxy. It turns out that regular waves of comet-like "Omega Clouds" from the galactic core spread out to find and impact with any collections of large, regular, non-naturally-occurring shapes they stumble across -- in other words, cities -- thereby ending any civilization they encounter.
** Two characters separately reach the same conclusion about the Omega Clouds' possible purpose, once it's discovered the clouds contain a device that can detonate them in a supernova-esque explosion. [[spoiler: They're '''art'''. And the galaxy is the canvas for a series of coordinate explosions that will all be visible simultaneously from someone looking down on the Milky Way due to the speed of light. Attacking alien civilizations is a bug, not a feature.]]
* In Frederik Pohl's ''Literature/HeecheeSaga'' series, [[spoiler:the Heechee had [[SealedGoodInACan hidden themselves away within Black Holes]] to avoid the attention of the Kugel (better known as [[NamesToRunAwayFromReallyFast "The Foe" or "The Assassins"]]), a race of energy beings that regularly wiped out emerging species. They eventually emerge because humanity is on the verge of getting the Assassins' attention.]]
* The Inhibitors of ''Literature/RevelationSpace'' were responsible for a ''lot'' of dead aliens.
* It's sort of a toss-up who you'd call the precursor and the precursor killer in Creator/LarryNiven's "Literature/KnownSpace" series. The Slavers (aka the Thrint) were a species of beings capable of psychically forcing any other life form to do exactly what they wanted. They weren't total idiots, but neither were they terribly clever, since once they made contact with other species, they had those other species do the thinking ''for'' them. The tnunctipun were the master technologists, but most of the artifacts from the Thrint era of the galaxy belonged to the Thrint rather than their tnunctipun slaves. When the inevitable war broke out, the Slavers' final act was to use a psychic amplifier to project the command "DIE!" to everything in the galaxy, killing everything sentient. So even though the Slavers are often billed as the precursors, they are themselves technically the ones who pulled the trigger.
* The eponymous Literature/{{Berserker}}s obliterated both sides of the war they were created to end and continued to scour the galaxy of life.
* ''Literature/TheHistoryOfTheGalaxy'' has two examples, although they're not 100% true to this trope, since the actual Precursor races are still around, while their killers aren't (yet). The Forerunners are a race of non-sentient PlanetEaters made up of proto-matter ([[spoiler:in fact, they're biological machines created by an EnergyBeing whose "programming" got messed up by a nova]]) who are responsible for the complete destruction of the [[FishPeople Delphons]] (the only Precursor race to completely die out) 3 million years ago, forcing the [[TheInsectoids Insects]] and the [[StarfishAliens Logrians]] to flee their space (inadvertently causing their enslavement by the [[HumanAliens Harammins]]). The much more ancient Evolgs and Emulotti were attacked by an expansionist race called the Shvergs billions of years ago. The Evolgs, being {{Energy Being}}s, helped the [[HumanAliens Human Alien]] Emulotti AscendToAHigherPlaneOfExistence, while their worlds were bombarded from orbit. The latest novels (going by the in-universe chronology) have the Evolgs, the restored Emulotti, the freed Logrians, and some of the Insects joining the human [[TheFederation Confederacy of Suns]] as equal members.

[[/folder]]

[[folder: Live Action TV ]]

* The Ori of ''Series/StargateSG1'' were revealed to have actually been the reason the Ancients traveled to the Milky Way Galaxy in the first place and planted he seeds of our own evolution. The Ancients were then driven out of ''our'' galaxy by a seriously nasty plague eons later, before ultimately succumbing to the Wraith in [[Series/StargateAtlantis the Pegasus Galaxy.]]
* It seems that most ancient empires in ''Franchise/StarTrek'' were destroyed by an unidentified allegiance of LaResistance. The ancient Iconian empire in ''Series/StarTrekTheNextGeneration'' was destroyed this way. In a reversal in ''Series/StarTrekVoyager'', the ancient empire of the Vaadwaur were tyrants and they sealed themselves when their enemies attacked.
** More recently, the Builders of the episode ''Prototype'' were wiped out by their own obsessive robot creations.
* In the backstory of the new ''Series/DoctorWho'', the Time Lords were all killed off by the Doctor himself, ending their war with the Daleks in mutual destruction: by that point, not only was the War causing an insane amount of damage, but the Time Lords had sunk so low that a "victory" for either side would spell bad news for the rest of the universe. The Daleks and the Master survived.
** He has since had to seal the deal on all three threats, although the Daleks keep popping up again due to JokerImmunity, and the Master has a long history of the same...

[[/folder]]

[[folder: Mythology ]]

* Zeus and the other Gods of [[Myth/ClassicalMythology Greek Mythology]] waged war against the Titans and won, although they didn't kill them (You see two every time you look up at the sky)
* Several gods in Near-Eastern religions (and one {{God}} in particular) acted as this with a GreatFlood, destroying the world that came before and allowing only one family of humans to survive.
* The Maya believed that their Gods created three worlds before this one, but were unsatisfied with the creatures therein and wiped the slate clean.
[[/folder]]

[[folder:Tabletop Games]]
* In ''TabletopGame/{{Warhammer 40000}}'', the Necrons (a race of undying skeletal androids) are the reason the Old Ones aren't around anymore, and the only reason the Necrons didn't end up ruling the galaxy was an outbreak of Warp entities.
* The job description of the TabletopGame/{{Exalted}}. There are ''some'' surviving Primordials, but that's because they surrendered when they realised how badly dead Primordials broke ''reality itself'', creating an entire world as a shadowy and dead mirror of Creation and damaging the formerly foolproof reincarnation system.
* In the TabletopGame/{{Alternity}} TabletopGame/StarDrive setting, the Stoneburners and Glassmakers were a faction of, respectively evil and good, precursors. The Stoneburners wiped out the Glassmakers in a massive war, but were themselves mostly killed off by the war. At which point, the I'krl, a servant race of the Stoneburners, took ''them'' out and became the new BigBad of the setting.

[[/folder]]

[[folder: Video Games ]]

* [[EldritchAbomination The Reapers]] of ''Franchise/MassEffect'' [[spoiler: and their [[TheManBehindTheMan master]], the Catalyst]] made this their ''profession''. They were Precursors to all the other Precursor races, [[AbusivePrecursors for the sole purpose of causing the genocide of them once they were advanced enough.]] Since they return during the games and can be killed, [[PlayerCharacter Shepard]] [[DidYouJustPunchOutCthulhu qualifies as a Precursor Killer him/herself.]]
** The Leviathan DLC [[spoiler: reveals the original Precursors, the Leviathans, who created the Catalyst in the first place and had most of their species converted into the first Reaper as the price]].
* The [[spoiler: Brethren Moons]] of ''VideoGame/DeadSpace3'' are implied to be the reason why [[AbsentAliens space is]] [[TitleDrop dead]].
* The Dark Precursors of ''Franchise/JakAndDaxter''.
* The player character gets to meet the wizard who nearly destroyed (accidentally) the Netherese Empire in ''VideoGame/NeverwinterNights: Shadows of Undrentide''. The guy who destroyed the rival Illefarn Empire is the main adversary in ''Neverwinter Nights II''.
* The Flood of ''Franchise/{{Halo}}'' is a race of parasitic alien spores that latch onto any sentient life with sufficient biomass, becoming more intelligent as they grew. The Forerunners were forced to use the Halos, a series of [[DoomsdayDevice doomsday weapons]] to wipe the galaxy clean of anything the Flood could infect, including themselves.
** It turns out that the Forerunner [[spoiler: wiped out their own Precursors]] as well. [[spoiler: And it turns out that The Flood is what results when decayed Pecursor remains are applied to sentient life.]]
* The Orz in ''VideoGame/StarControl II: The Ur-Quan Masters'' seem like fairly harmless [=Cloudcuckoolander=]s but are actually part of an EldritchAbomination.
** ''Star Control'' also has the Sentient Milieu being brought down by the Dnyarri, then the Dnyarri being brought down by their Ur-Quan slaves (and then one branch of the Ur-Quan exterminated at least some of the surviving Milieu species). The reason for the actual, Capital-P Precursors in the universe disappearing remains unknown, despite what [[FanonDiscontinuity some people claim]] happened in ''Star Control III''.
*** For extra twist points, the Sentient Milieu's enslavement began when an Ur-Quan explorer stumbled upon the Dnyarri's homeworld.
* The Xel-Naga from ''Franchise/StarCraft'' created races as a means to perpetuate themselves, and created the mentally advanced Protoss, and biologically adaptive {{Hive Mind}}ed Zerg, two species that would eventually join to become the new Xel'Naga. [[SpannerInTheWorks Except]]... Some {{Jerkass}} EldritchAbomination known as the Fallen One threw a monkey wrench in their plans, by subtly altering the Zerg into genocidal killing machines. The Zerg then promptly became Precursor Killers.
* The Shivans wiped out the Ancients in the ''[=VideoGame/FreeSpace=]'' backstory, and it's strongly implied that they've done the same to any other civilisation that spread too far throughout the galaxy and/or through Subspace.
* The Space Pirates seem to be to blame for wiping out most of the remaining Chozo in ''{{Metroid}}''. What the Space Pirates didn't get, phazon did.
** The Chozo are implied to still be out there ''somewhere'', though. With phazon gone, most were presumably able to return to [[AscendedToAHigherPlaneOfExistence a higher plane of existence.]]
** Gorea wiped out the Alimbics of ''Metroid Prime: Hunters.''
* In the ''VideoGame/MightAndMagic'' series, it's [[CanonDiscontinuity revealed]] that the alien Kreegan (which the galaxy's less advanced races mistake for demons) were the ancient enemies of the Precursors. Since the Kreegan are still around, and the Precursors are not, it seems pretty clear what happened (although the good ending of ''M&M VII'' seems to suggest that some surviving Precursors moved to another galaxy or something).
** According to ''M&M VIII'', the Precursors are still around, fighting the Kreegan on other fronts. There are still elements of this trope, though- it was the Kreegans' invasion and sheer numbers that caused the collapse of the old system and the isolation of numerous systems from the Precursors.
* ''[[VideoGame/AssassinsCreedI Assassin's Creed]]'' has an antagonist mentioning his belief that the "artifacts" are left over from "those who came before," but ''VideoGame/AssassinsCreedII''[='=]s ending reveals that [[spoiler:primitive humanity's sheer numbers were enough to overwhelm the precursor race]]. However, it's strongly implied that [[spoiler:the playable characters' bloodline is genetically descended from [[HalfHumanHybrid a human-Precursor hybrid]], and the Facebook game ''Project Legacy'' briefly implies that Giovanni Borgia is part of a hybrid line possibly going as far back as Marcus Junius Brutus the Younger. Yeah, ''[[EtTuBrute that one]]'']].
** It turns out that [[spoiler:a solar flare]] was the true Precursor Killer. [[spoiler:The surviving precursors were too few to repopulate their race, but they worked together with their former slaves/enemies to rebuild the world and took steps to ensure that it would survive the next inevitable solar flare. The existence of hybrids and humanity remembering the precursors as deities show that the precursors and humanity made peace with each other before the former died out.]]
* In ''VideoGame/BlueDragon'', [[spoiler:Nene's "pet" Deathroy aka [[NamesToRunAwayFromReallyFast Destroy]]]] was the biomechanical living weapon that wiped out the Ancients' civilization.
* The Juno to [[spoiler:humanity]] in ''VideoGame/{{Solatorobo}}''. In an interesting twist, they actually only made the suggestion and provided the kill program. The [[spoiler:humans]] are the ones who made the final suicide call, seeing it as the only way to end their wars and give the planet a chance to recover without them.
* In ''Franchise/MasterOfOrion'', the Orions and Antarans are the two main Precursors who fought a huge war well before the first game. The Orions won by trapping the Antarans in a bubble dimension, but the resulting disruption to FTL travel resulted in them becoming their own PrecursorKillers. The second game reveals that the Antarans survived and return to conquer the galaxy, but by the start of the third game they've nearly been wiped out by one of their own experiments - a sentient, parasitic biological weapon that would presumably have become a Precursor killer had the series continued. As it is, the most likely Precursor killer is the player, although this is not strictly necessary to win the game.

[[/folder]]
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