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, all of which have different active skills. Their Noble Phantasms also have different names and all deal damage like the original Black Irisviel, but their secondary effects vary. Those variations are Fire Iri, Water Iri, Wind Iri and Earth Iri.