Complex sentence: Azazeal is revealed to be the leader of the biblical Nephilim, fallen angels, and he claims to be in love with Cassie, despite dalliances with schoolmistress Jo Watkins and Cassie's own mother, which may have contributed to her mental illness.
Split into simpler sentences: Azazeal is revealed to be the leader of the biblical Nephilim, fallen angels, and he claims to be in love with Cassie. He has had previous relationships with schoolmistress Jo Watkins and Cassie's own mother, which may have contributed to her mental illness.