Black Lake (3.5e Encounter)
From D&D Wiki
 Encounter: Black Lake
- CR 13
- Integration Perfect for a magical dungeon of any sorts!
- Prep Time Just read the creature listing :)
- Play Time Short (20 minutes)
A wizard has set up a trap. There is a room 20 feet wide and 60 feet long. In the middle is a pit 20 ft. by 20 ft., but it is only 20 feet deep. At the bottom of the pit is a bubbling thick black liquid. A successful Knowledge (dungeoneering) check DC 30 identifies it as an Elder Black Pudding.
At the far edge of the pit is a Wall of Force (which is invisible), so players attempting to jump the pit will find themselves hitting the wall and falling right into the pudding!
The pudding always attacks the nearest person, but will wait until someone falls on to it. If the party attacks it directly, it will climb right out of the pit (20ft. climb speed) and attack the nearest target.
13,000 gp worth of stuff, though this can be a great "last room" of a dungeon, so you can put a stockpile of stuff here.