I have zero recollection of writing this, but it’s in my notes?
What was it? Write down ten things you’d find there. Write down how five of those don’t work like they should. Write down why no one uses it any more.
Now pick whatever monsters you want to challenge players. Quick, answer how its here if that doesn’t make sense, and Why if “the surface world hates me and I don’t want to die” isn’t good enough. Now, seriously, grab a copy of the pyramid of needs. Rewrite as necessary to suit weird monsters’ weird psychology. (Dragon’s basic needs include Treasure, and the like). Scratch through a handful of them that it doesn’t need — now wherever it shows up in the dungeon, that place is vital to fulfilling the remaining, unscratched needs.
In play, hold yourself to these things.
Describe your ten things regularly, as appropriate, and work in how they’re corrupted or don’t work anymore.
When your monsters show up, remember their needs define their lifestyle — what they have is reflected in what they carry, their surroundings, and the like. If they’re not in their lair, pick a need, and that place you find them in is vital to fulfilling it — I guess the room is now a Kitchen, so populate its surroundings accordingly.