Complex sentence: Boyd's mental deterioration while in prison was evident to Holmes, and he says that it was after his release that Boyd began to identify as Abba Mobus Abadon, derived from "God" and the Hebrew word for "a bottomless pit".
Split into simpler sentences: Boyd's mental deterioration while in prison was evident to Holmes. He says that it was after his release that Boyd began to identify as Abba Mobus Abadon, derived from "God" and the Hebrew word for "a bottomless pit".