What did you do, Ray?!!
Well, now you've done it. Save vs Mental Spells, or Roll a d12 to determine what form pops into your head (from some near-forgotten childhood tale, or elsewhere). If everyone makes the Save, the Form will be chosen by the DM.
1. The classic Stay-Puft Golem. Deceptively innocuous looking, massively huge, dangerously viscous, highly flammable (no Save/dbl damage vs. fire)
2. A large and moving Torb. Much worse than the immobile variety, for obvious reasons
3. A giant Slor, its gullet like a roaring furnace, enough to make a Balrog jealous
4. The last creature which caused a successful Fear among the Party
5. The last creature which slew a Party member
6. The worst Thing on the local Random Encounter table
7. A Shoggoth
8. A Spawn of Shog-Gnargoroth, possessed of odd magic/mutant abilities http://tengoldpiecegems.blogspot.com/2011/01/super-simple-random-beasties.html
9. The goateed Evil Twin of the most powerful Party member
10. Roll on Dungeon Lvl X table
12. Players get to vote on an entry from this table