Complex sentence: Some Byzantine sources, however, imply that he had already rebelled before Leo's death, and also claim that he pretended to be Emperor Constantine VI (r.780--797), but the validity of this report is questionable.
Split into simpler sentences: Some Byzantine sources, however, imply that he had already rebelled before Leo's death. Byzantine accounts also claim that he pretended to be Emperor Constantine VI (r.780--797), but the validity of this report is questionable.