Together with his brothers, Zeus and Hades, Poseidon cast lots for sovereignty over the world. Zeus took the sky, Hades the underworld, and Poseidon the sea. Poseidon's emblem was the trident, a three-pronged spear with which he could strike the ground, causing springs to emerge. His most significant consort, of whom there were many, was the sea nymph Amphitrite, and together they had a merman son, Triton. When Poseidon and Athena competed for sponsorship of the city of Athens, Poseidon struck the Acropolis with his trident, and a salt spring burst forth.Athena, however, planted an olive tree and was chosen by the Athenians as their civic deity. In revenge, Poseidon flooded the plain of Attica, on which Athens stands. Poseidon was the father of Theseus and of many other heroes, but some of his children were more than human. In one myth Poseidon courted the goddess Demeter, but his love was not reciprocated. To avoid him she turned herself into a mare. Poseidon, however, became a stallion and covered her; their offspring was the magical horse Arion. Unlike other primordial water gods, Poseidon is a fully-fledged personality, and one separate from the natural phenomenon he controls.
by Barry Powell, 30 Second Mythology