Deicide

Deicide is a Challenge in Risk of Rain 2. Completing it unlocks the Rare item Brainstalks.

Defeat an Elite Boss on Monsoon difficulty
While actually killing the boss is hard enough on its own, spawning an elite boss is a challenge in and of itself. It may take multiple Shrines of the Mountain to be activated for an Elite Boss to spawn in the earlier difficulties, as just one or two will usually just spawn multiple regular bosses.

An Elite Boss can be told apart from regular by their hue, as well as their name: Elites have a descriptor before their names.

The slain enemy must be a boss. An Overloading Greater Wisp, for example, is not a boss-enemy, regardless of whether or not it spawned from the teleporter.

"Horde of Many" encounters, in which many powerful enemies are spawned including one elite, do not count despite their inclusion of an elite. The boss in question must be the boss spawned by the teleporter; it cannot be a boss-type enemy that has randomly spawned on the map.

When attempting to unlock this achievement in multiplayer, be aware that the achievement seems to only be unlocked for the person who deals the killing blow to an elite boss. Whether or not this is a bug is unknown.

Trivia

 * "Deicide" is the crime or act of killing (a) god.

Deicide