Complex sentence: He is convinced by Mack to use his powers to help people and decides to stay in the Lighthouse while his allies head to the surface though Mack and Yo-Yo decide to stay to help him.
Split into simpler sentences: He is convinced by Mack to use his powers to help people. He decides to stay in the Lighthouse while his allies head to the surface though Mack and Yo-Yo decide to stay to help him.