Apsu is one of the Primordial Gods, created by the Drylon over five billion years ago. Together with Tiamat, they gave birth to the Sumerian pantheon. When the younger generation of gods rebelled against them, Apsu was killed with a Drylon weapon.
Reality warping: Apsu could manipulate matter and energy at a galactic scale, take any form and size he desired, was completely invulnerable, and could teleport anywhere in the universe instantly. He may even have been able to warp the laws of physics, but he was still vulnerable to Drylon weapons.
He has never appeared in the series. His story was described in Beyond The Impossible #159, "When the sky above was not named"