Poseidon is the Greek God of the Sea, and as such he had responsibilities such as protecting and/or killing fishermen and sailors perceived to be a threat to the ancient Greeks. Furthermore, Poseidon oversaw earthquakes, was the patron of horses and was also thought to be involved in the presence of epilepsy.
Among the many Greek gods, Poseidon is one of the most important, both historically and in mythology.
After defeating the Titans, Zeus, Poseidon and Hades all drew straws to determine the realm over which they would rule. Zeus became ruler of the heavens, Poseidon the sea and Hades the underworld. Since the sea was such an important part of Greek trade and food, Poseidon was one of the most important gods. The Ancient Greeks believed that if Poseidon were displeased with a sailor or fisherman, he would use the sea or an array of fierce underwater creatures to kill them. Therefore, they often offered sacrifices to please Poseidon before going on an expedition or going out to fish. As Poseidon was the patron of horses, many Ancient Greeks drowned stallions as a sacrifice to please him.
Poseidon was also heavily involved in the Trojan War and assisted the Trojan king in fortifying the defenses of Troy. However, the king did not reward Poseidon for his assistance, so Poseidon sent a Kraken to attack the city and punish the king.