Poseidon violated Medusa in the most horrific way when she went to check up on Athena by the orders of the Elders. She was in love with Poseidon, and easily fell victim to his assault. Because the deed was done in Athena's temple, she was punished by the goddess who turned her into a gorgon. Since Poseidon was mortal with divine powers, he eventually died.
Notes and Trivia
In Soul of the Bride, Piper mentioned that Poseidon, Zeus and Hades drew lots to decide how to divide the world and who would rule over those parts. While Zeus won ownership of Mount Olympus and Hades became the lord of the Underworld, Poseidon became the ruler of the sea.