Black Knight is a demon in the series.
The black knight is a famous stock character used in pop culture and literature. It was made famous with its several appearances in Arthurian legend. One tale has him tie his wife to a tree and duel Percival for sharing rings, only for Percival to defeat him and state it was innocent. Another black knight murdered his mother after his father's ghost told him that his wife murdered him, while a different black knight was killed by Gareth as he was traveling.
Kyuuyaku Megami Tensei