Complex sentence: An Irish priest arrives soon after, and performs the exorcism that purges the demon from Maggie, but the demon ends up possessing Bart, which he fearfully regrets, declaring that Bart has the evilest soul he's ever seen.
Split into simpler sentences: An Irish priest arrives soon after, and performs the exorcism that purges the demon from Maggie. But the demon ends up possessing Bart, which he fearfully regrets while declaring that Bart has the evilest soul he's ever seen.