Complex sentence: He was recruited into the ranks of the devils by Asmodeus and quickly climbed through the ranks of the fiends, assuming control of the seventh layer to become one of the most powerful devils in Baator.
Split into simpler sentences: He was recruited into the ranks of the devils by Asmodeus. And quickly climbed through the ranks of the fiends and assumed control of the seventh layer becoming one of the most powerful devils in Baator.