Poseidon, brother of Zeus, is the Greek god of the sea. He’s often shown holding a trident. Earthquakes are also accredited to Poseidon the Earth-Shaker (or blamed on him).
One of Poseidon’s children is a one-eyed giant (Cyclops) called Polyphemus. Polyphemus captured Odysseus and his men on their way back from the Trojan war. After Polyphemus ate a few of the men, Odysseus blinded him by means of a cunning trick. Unfortunately for Odysseus, he has to sail home over the seas which are ruled by the Cyclops’s angry father, Poseidon.
Poseidon gives Odysseus no end of trouble. The other gods won’t let him kill Odysseus, but that does not stop him sending storms and monsters (like Scylla) to hamper him and sometimes to kill his men.