Complex sentence: Percy begins training to use his demigod powers, which include water manipulation and using water to heal, and meets other demigods including Annabeth Chase, daughter of Athena, and Luke Castellan, son of Hermes.
Split into simpler sentences: Percy begins training to use his demigod powers, which include water manipulation and using water to heal. He also meets other demigods, including Annabeth Chase, daughter of Athena; and Luke Castellan, son of Hermes.