Complex sentence: Before Wendy and her aunts have a chance to escape, Desmond arrives and tries to cast Wendy into the Mystic Abyss to kill her, but Casper dives in to save her.
Split into simpler sentences: Before Wendy and her aunts have a chance to escape, Desmond arrives. He tries to cast her into the Mystic Abyss to kill her, but Casper dives in to save her.