Zombie Portal

Also known as a Portal, the Altar exists in most dungeons and spawns undead. When discovered, the altar activates and spawns undead at a sometimes alarming rate. Early on, finding one spells death unless fast action is taken to block it off completely.

A picture:

This Altar however is already blocked off. Also note that the Altar spawns most kinds of undead, leading to figures like 15 skeletons, 20 zombies and 7 wraiths spawned after the initial discovery.