Rotfiend blood is an alchemy ingredient in The Witcher 3: Wild Hunt that can be dismantled into monster blood. It is needed to craft the following items:
It is a guaranteed drop from rotfiends, their monster nests and devourers who guard them, and scurvers in the Blood and Wine expansion. It can also be purchased from Otto Bamber at the Herbalist's Hut northeast of Oxenfurt.