Sometimes Its Boring in the Dungeon

d12
1. Entire level once an ancient library haunted by undead entities consumed by a lust for research, 96% of collection devoted to genealogical records of the slugmen
2. Diabolical system of teleportation glyphs sends hapless party from 10' corridor to 10' corridor in continuous loop, 1 in 6 chance per glyph of return to the surface so they can return to headquarters in disgust
3. Since the party's last ill-fated visit dungeon gentrified by lawful monsters, former deathtraps and torture chambers converted into boutique shops and cafes (of evil)
4. The level subject to refurbishment by unionized humanoid work crews, mandatory break times enforced, no fighting allowed, dungeon traffic rerouted to little-used thoroughfares devoid of monsters, treasure
5. Level empty except for scads of semi-precious stones scattered everywhere, requires hours of tedious labor to gather but adds up to a decent sum, process gives PCs a chance to share their backstories in full detail
6. Incredibly long monster queue for compulsory annual audience with the Dungeon Overlord winds throughout entirety of level, monster patience still holds, but it could get less boring in a hurry
7. Dungeon loaded with tiny humanoids devoid of any tactical sense and scarcely a hit point between them, they just keep coming in wave after stupid wave
8. After depopulating the level in an indescribable bloodbath, the succubus just wants to play chess. Really, that's it.
9. Monster ballet choreographed by vampire aesthete with large bribery budget performed in deep level amphitheater, thunderous footfalls, musical accompaniment of evil audible from dungeon entrance, rooms empty
10.  In an unprecedented executive order, Dungeon Overlord issued valueless paper currency, all precious metals, gems, etc., impounded in impregnable super-bunker, economic collapse imminent, chests and coffers offer only stacks of poorly printed notes, guards no longer give a shit
11. Supremely powerful, typically hateful eldritch deity is the only occupant of the level, gives chase, moves incredibly slowly, bellows out resounding recap of its own eons-long backstory
12. All levels but the first completely cleared by rival party via advantageous secret entrance, already drunk back at the Ol' Adventurers' Saloon, chomping cigars and telling satirical anecdotes at the PCs expense