Dark heart

, as well as the other variants (Dark brain, Dark intestine), will randomly appear on newly delivered corpses. Obtained after progressing far enough through the Inquisitor quest-line.

Quest
Snake asks you to bring him a for his final ceremony.

This is requested by the Witch Clotho to heal Koukol's curse.

The Inquisitor needs evidence for the presence of an occult in the town to convince the king of the inquisition's plans. For this he asks you to bring him one, one Dark brain and one Dark intestine.