King Poseidon was the first king of the drowned Atlantis.
King Poseidon ruled Atlantis when the Old Ones came. Knowing he could not defeat them alone, he drew all of the mystical energy in the world and forged it into a trident. He managed to defeat the Old Ones and the most powerful among them, Icthultu. But because he had taken all the mystic energy, he also drained the power that kept Atlantis afloat. His city drowned.