Poseidon is one of the 'Big Three' and the god of the sea. He also got a lot of half god children, like Zeus. His attribute is his trident. He's the enemy of Athena, the goddess of wisdom. An example of their hostility is the city of Athens; they both wanted to become main god of the new city, and after a while, they got to an agreement. They would each give something new to the city and after that, the citizens would choose. Poseidon came up with a salt water fountain and Athena brought them the olive tree. The citizens couldn't see where the fountain would come in handy, and they chose Athena. Another example is Medusa, who ever was the most beautiful girl in Greece. But Poseidon took her as his and he made a date to go to Athena's temple. This was such a humiliation for Athena, she turned Medusa and her sisters into monsters who could only frighten people to death.