Precursor Killers

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 Lost Superweapon to be found that the precursors weren't able to finish in time to stop the Precursor Killers.

Often this is a Sealed Evil in a Can that was somehow stopped with the last actions of the precursors. Compare Abusive Precursors and Turned Against Their Masters.

Fan Works

 * In With Strings Attached, 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.

Film

 * Transformers: Revenge of the Fallen 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.

Literature

 * The entire tragic backstory of the elves in the 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 Jack Mc Devitt'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.
 * In Frederik Pohl's Gateway/Heechee Saga series, *
 * The Inhibitors of Revelation Space 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 Larry Niven's "Known Space" 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.

Live Action TV

 * The Ori of Stargate SG-1 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 the Ori eons later, before ultimately succumbing to the Wraith in the Pegasus Galaxy.
 * It seems that most ancient empires in Star Trek were destroyed by an unidentified allegiance of La Résistance. The ancient Iconian empire in Star Trek: The Next Generation was destroyed this way. In a reversal in Star Trek: Voyager, 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 Doctor Who, 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' by 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 and the Master keep popping up again due to Joker Immunity.

Mythology

 * Zeus and the other Gods of Greek Mythology waged war against the Titans and won.

Tabletop RPG

 * In 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 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.

Video Games

 * The Reapers of Mass Effect  made this their profession.They were the original Precursors, who became Precursors to all the other Precursor races, for the sole purpose of causing the genocide of them once they were advanced enough.
 * Of course, once the Reapers start dropping, Shepard becomes a Precursor Killer himself.
 * The Dark Precursors of Jak and Daxter.
 * The player character gets to meet the wizard who nearly destroyed the Netherese Empire in Neverwinter Nights: Shadows of Undrentide. The guy who destroyed the rival Illefarn Empire is the main adversary in Neverwinter Nights II.
 * The Flood of Halo; 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 doomsday weapons to wipe the galaxy clean of anything the Flood could infect, including themselves.
 * It turns out that the Forerunner  as well.
 * The Orz in Star Control II: The Ur-Quan Masters seem like fairly harmless Cloud Cuckoolanders, but are actually part of an Eldritch Abomination.
 * Star Control also has the Sentient Milieu being brought down by the Dnyarri, then the Dnyarri being brought down by their Ur-Quan slaves. The reason for the actual, Capital-P Precursors in the universe disappearing remains unknown, despite what some people claim happened in Star Control III.
 * The Xel-Naga from Starcraft created races as a means to perpetuate themselves, and created the mentally advanced Protoss, and biologically adaptive Hive Minded Zerg, two species that would eventually join to become the new Xel'Naga. Except... Some Jerkass Eldritch Abomination 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 Free Space 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 a higher plane of existence.
 * Gorea wiped out the Alimbics of Metroid Prime: Hunters.
 * In the Might and Magic series, it's 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.
 * Assassin's Creed has an antagonist mentioning his belief that the "artifacts" are left over from "those who came before," but Assassin's Creed II's ending reveals that . However, it's strongly implied that.
 * It turns out that was the true Precursor Killer.
 * In Blue Dragon, was the biomechanical living weapon that wiped out the Ancients' civilization.
 * The Juno to in Solatorobo. In an interesting twist, they actually only made the suggestion and provided the kill program. The  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.