Complex sentence: One of his last memories before escaping Purgatory is the Leviathans chasing him this also could be explained by the fact that, being monsters, when they die they return to purgatory.
Split into simpler sentences: One of his last memories before escaping Purgatory is of Leviathans chasing him. This could also be explained by the fact that, being monsters, when they die they return to purgatory.