Nega-Poseidon is the Greek god of the seas and a boss from God of War III.
Along with the rest of his godly brethren, Poseidon battled the Titans in the devastating Great War. He helped his brother, Hades, bring down the Titan, Atlas, striking him with his lightning magic, weakening him until the god of the dead could tear out his soul. After the defeat of the Titans at the hands of Zeus with the Blade of Olympus, Poseidon gained dominion over the seas.
God of War
After the Hydra emerged, it began terrorizing the Aegean Sea and challenging Poseidon’s dominance. During one of its attacks on a fleet of ships, Kratos appeared and began battling the Hydra’s lesser heads. As he continued fighting the monster, Poseidon appeared before the Ghost of Sparta and gave him the magic of Poseidon’s Rage, which would allow Kratos to unleash the god of the sea’s lightning magic from his body. The Spartan succeeded in slaying the Hydra, allowing Poseidon to retake his control of the seas.
God of War: Ghost of Sparta
Poseidon was the ruler of Atlantis. When Poseidon found out that Kratos was heading to Atlantis, he set forth Scylla (and other creatures) to prevent Kratos from sinking Atlantis. After Kratos was successful in sinking Atlantis, Poseidon eventually found out. While Kratos was sailing toward the Sunken Atlantis, Poseidon sent an army to attack Kratos. After they were defeated, Poseidon decided to strike the ship with lightning. When Kratos reached Atlantis, Poseidon warned Kratos that he will regret on what he has done, with Kratos ignoring him.
God of War II
After Zeus’ defeat at the hands of Kratos, Poseidon attended a meeting on Mt. Olympus that the king of the gods had called. Along with the other remaining gods, the god of the sea witnessed the Ghost of Sparta lead the Titans in an attack on the mountain.
God Of War III
When Kratos and the Titans were climbing up Olympus, Poseidon quickly leapt to the defense of Mt. Olympus and dropped from the top of the mountain, smashing through Epimetheus’ chest and landing in the ocean at the bottom. Poseidon then summoned the Hippocampis, which began taking hold of the Titans, including Gaia. The sea god eventually emerged from the seas in a watery construct modeled after himself with Hippocampis sprouting from the bottom. Kratos was unphased by Poseidon's new form; he was even unafraid of Poseidon's warning that Kratos had defied the gods for the last time. Poseidon used the Hippocamis to hold Gaia, while he attacked the Titaness and Kratos. After a terrible battle, the Ghost of Sparta was sent flying through the center of the construct, colliding with his body and knocking the sea god out of his watery form. As Kratos advanced on Poseidon, he warned Kratos that Olympus’ destruction meant the world’s destruction. The Ghost of Sparta did not react and proceeded to brutally beat on the sea god, before finally finishing Poseidon by gouging out his eyes with his thumbs and snapping his neck. Kratos threw his body off the mountain and, when it collided with the seas, an enormous tidal wave swept across Greece, leaving almost nothing above water.
Later on, While Kratos was in Poseidon's Camber, he found note. It was revealed that Poseidon had been suspicious of Zeus' behavior and was also interested in Pandora’s power.
Role in the Spongebob series
Like Hades the Monstrous, he's still stuck in limbo in Kingdom hearts, but it's implied Nega-Zeus would long for his brothers and fellow gods soon enough when he needs to add more power into the Guild of Thunder.