Complex sentence: A common version of her story is that Apollo gave her the power of prophecy in order to seduce her, but when she refused him, he gave her the curse of never being believed.
Split into simpler sentences: A common version of her story is that Apollo gave her the power of prophecy in order to seduce her, and then cursed her when he failed. Athena cleaned the ears of Tiresias When Cassandra refused Apollo's attempted seduction, he gave her the curse of never being believed.