[Suzerain of Sheol]

One of our favorite spells was Unliving Weapon from the Book of Vile Darkness. Makes an animated undead explode when killed. Endless possibilities.

As for traps, one of my worst (best) was a teleportation circle into the bottom of an underground lake of acid covered in permanent antimagic fields, with a clan of Black Slaad living on the shore to pick off any survivors. This was in another epic dungeon we ran, have to be tough on players at that level.