Most people believe that Isley defeated Priscilla when she went on a rampage in the North and made her his lover. However, this was later proven untrue - despite initially holding the upper hand, Isley had nearly half his torso torn off by Priscilla, who in contrast regenerated from her injuries at phenomenal speed.
For unknown reasons, Priscilla suddenly reverted back to her human form in the midst of battle. Realising that her Awakening had caused her mind to regress, Isley responded by returning to his human form as well, and swore allegiance to Priscilla. His exact motivations remain unknown.