Complex sentence: Percy finally defeats it with the help of Tyson, Annabeth, and Clarisse, and Luke Castellan suddenly appears and confronts Percy, trying to convert the son of Poseidon to his cause.
Split into simpler sentences: Percy finally defeats it with the help of Tyson, Annabeth, and Clarisse. Following this, Luke Castellan suddenly appears and confronts Percy, trying to convert the son of Poseidon to his cause.