In the middle of the rise there will be a crude shrine with a nephalem victim on it, slowly exsanguinated by Blood Cultists. Among them, Hematic Father Feldman, who is invulnerable and inactive at the start of combat.
Upon killing every single Cultist, Feldman will summon more and engage in battle himself. Both he and his followers must be slain.
When all Cultists are dead, the last thing to do is to put the tortured warrior out of his misery. Interacting with the shrine will grant him peace and end the event.