Dungeon Extinction Events

d12
1. Puny humanoids on level one, driven by obsessive trap-mania of master thief-assassin clan boss, built so many deadly traps all the other denizens are dead, many of their own killed daily just getting from here to there
2. Normally fecund medium size humanoids on level two rendered permanently sterile by exposure to gas released capriciously by sorcerer from level four, population facing oblivion with characteristic savagery
3. Giant snakes lay about in messy heaps, victims of newly arrived giant rats w/toxic, mutated body chemistry, garden variety giant rats already wiped out, reeking snake carcasses attracting, subsequently poisoning other dungeon vermin/scavengers
4. Everything that breathes (especially the sorcerer) on level four dead after experimental fireball fusillade spell ate up all the oxygen
5. Once-teeming skeletons on the tomb level, already dead, come unglued due to highly transmissible undead virus attacking their eldritch connective tissues, picked up from visiting cosmopolitan lich, bones lie about in random heaps, some still animate, impotent but for the ability to provoke heebie jeebies,
6. All entrances to level five sealed by custom hold portal spells, monsters make war of frustration upon one another, trolls doing well at the moment, chaos cultists still have ace up their sleeve
7. Level six recently visited by That Other Adventuring Party via teleportation, carnage everywhere, treasure no where, vulgar graffiti viciously lampoons PCs
8. Level seven subject to catastrophic flooding event precipitated by meandering cthonic worm discovering underground lake, black pudding population enjoying the change of pace, everything else floating face down
9. Sudden rise in lava levels in the fire caverns accompanied by release of lethal volcanic gas brought an end to production at the dwarf foundry, their giant iron guardians waist deep in cooling lava, magic swords sticking out of the flow here and there
10. Extra-planar entity, duties to its irresponsible summoner fulfilled, steadily clear-cuts the fungus forest in feeding frenzy, seems insatiable, entire mini-ecosystem in tailspin
11.Dwarf insurgents detonated experimental explosive device on mine level, eldritch fallout results in neutron bomb-like effect, everything dead, treasures intact, as are the vampires in their secret crypt, no one should go in there for like fifty years
12. The dragon had a paranoid melt-down after recent treasure inventory showed significant stock shrinkage, roams about issuing accusations followed almost immediately by fiery breath, claw-claw-bite, only the really stupid monsters left but they're next