The Black Grail is the corrupted aspect of Justeaze Lizrich von Einzbern, wearing the appearance of Irisviel. She is an NPC-only Avenger Class Servant who appears as the final boss of Fate/Accel Zero Order.
This Servant has several variations. Their Noble Phantasms have different names, which all deal damage like the original Black Irisviel, but have different secondary effects. They also have their own skills.