The Cultist’s Blood is a Legendary Crafting Material found in Act I. While there are a few sources it drops from, you want to farm the one that spawns permanently, not a random target, right?
Go to Halls of Agony Level 1 and kill Cultist Grand Inquisitor. He spawns every time you launch a game.
The Cultist’s Blood is used to craft a Legendary Dagger: Blood-Magic Edge. The Punyal required to craft this weapon can be farmed at the same location from the Torture Tools (weapon racks).