Ok, so what are a medusa, a vampire queen, a fat unconscious demon, and a goblin alchemist all doing in the same dungeon?
You could just throw all those monsters in there, but having a logic to it all actually makes it easier to write. If the monsters are there for a reason then it's easy to decide how they'll react to unexpected behavior on the part of the PCs.
So here's the logic:
The demon is from a gazillion years ago. He and his brothers once ravaged the Earth--as colossal demons will.
So along comes a horrible necromancer. He's horrible, but he likes living and not being stepped on, so he recruits some medusas to walk around and turn the demons to stone.
So the demons are turned to stone. The last demon is blind so he can't be turned to stone, but they stop the last demon anyway with some kinda black magic.
So then the problem is imprisoning him. So they slice all the petrified demons into blocks and imprison the last demon underground inside a prison made from his brothers.
The horrible necromancer builds a whole underground complex around the imprisoned demon--with a very nice library. He can use it to leech off arcane power from the demon for various nefarious purposes. Plus the demon has a sort of subtle evil influence over local events even though he's chained up.
The necromancer's got a thing with one of the medusas (facilitated by some sort of petrification-proof charm, I guess), she lives there too.
Sooner or later, sometime in the past, what with all these adventurers going around adventuring all the time in D&D-world, the necromancer gets neutralized by some do-gooders. (Some PCs in some campaign some guy I've never heard of who lives in Nebraska ran in 1983, probably.)
He is then entombed, in a semi-living state, in his own dungeon. In a secret location.
His medusa girlfriend sticks around, statue-fying occasional visitors, living the lonely life of a medusa. Collecting art, etc.
Thousands of years go by, eventually the entombed necromancer develops enough power to place a necromantic curse on an ant or small spider. The ant will turn the first person it bites into a vampire.
In the hopes of creating enough trouble in the outside world that someone might stumble on his tomb, the necromancer sends the ant out to cause trouble in the outside world.
It bites a hapless halfling.
So the halfling, she becomes a vampire, and raises a ruckus with enslaved fellow halflings.
They all move underground so she can stay out of the light--the complex is naturally the place they go since it's close to where they live.
Meanwhile, a sphinx--being, like all her kind, a scholarly type--has been looking through the library (it's a very nice library). When the vampire queen finds the sphinx in there, she captures her and locks her up in the lowest dungen levels so she can ask her questions, if need be.
Now the medusa isn't happy about this, but anybody who comes near her door she turns to stone, so she's willing to tolerate the halflings being on the other side of the complex for a little while.
The vampire and company have been there for like a month when the posse shows up.
The goblins just recently showed up because they got lost and a cave collapsed behind them when they were in the area. They've been here about a week.
Then, as usual, you got brainless wandering monsters that live udergorund and eat stuff.
And then there's the posse, who just showed up.
Makes sense, right?