Once the Olympian gods were in power, they decided that it would be best to divide up the world into different parts and choose who would rule over which. Even though Zeus was the overall leader and winner of the fight against Cronos, he let his brothers have power too because he thought it would be more fair.
One day he gathered them on Mount Olympus and they drew lots to see who would rule what.
Three pebbles of different colors were chosen. Each pebble represented a different place. Whomever chose the white pebble became King of the gods, whomever chose the gray pebble would be god of the seas, and whomever chose the black pebble would be god of the underworld. The pebbles were shaken up in a helmet and Zeus drew. He pulled out the white pebble. Then Poseidon drew the gray pebble. Finally Hades went and drew out the black pebble.
After this each god took responsibility because they all knew the importance of each place.
Zeus became king of the gods, which, in turn, made him the most powerful.
Poseidon became god of the seas, which, gave him control over everything related to the sea
Hades became god of the underworld, which, gave him control over the souls that were sent to the afterlife.