Evil Urns are a type of Container found only in the frozen lands around Mount Arreat. They have a function similar to Monster Shrines, but instead of transforming the nearest monster into a Champion monster, they summon a Champion, Ghostly, Fanatic, Berserker or Unique monster and its minions.
These appear in a range of half a screen of the urns, and sometimes, even on top of the player character. They can be trapped just like other containers, in addition to spawning monsters. When Evil Urns are found near level entrances/exits they will only drop gold or items, since no random monsters can spawn in those locations. For example, there is always an Evil Urn found in the Arreat Plateau, near the entrance to the Crystalline Passage, beside Thresh Socket, which only drops gold stacks.
They randomly select a monster from Normal and Nightmare/Hell spawn pools respectively, which means even if a monster on Normal can't have an Unique or Champion in particular location, this urn can still spawn a boss of that monster.