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 appeared 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 and also own skills. Those variations are Fire Iri, Water Iri, Wind Iri and Earth Iri.