The Sea King is the sea-faring member of Earth-3's Crime Syndicate and he exists as the villainous counterpoint to DC Comics' Aquaman.
Justice League: Trinity War
The Sea King was the first member of the Crime Syndicate to arrive on the Earth of the main DC Universe, following the Outsider's opening of Pandora's Box. Due to the strain of interdimensional travel, he perished upon arrival.
Justice League Dark
The Sea King reappears in Justice League Dark, with no memories of his past. Filled with an uncontrollable rage, he finds himself up against John Constantine, Swamp Thing, and Nightmare Nurse. Attempting to swallow them within an ocean wave powered by the spirits of Nan Madol, Sea King is dropped by a mystical severing spell. It is revealed that Justice League Dark member Deadman had jumped into Sea King's deceased corpse after the Syndicate's assault. Due to the King's echoing traces of evil, Deadman lost control of the body and fell prey to an amnesiac state.
Using his magicks, John Constantine has cleared out all remnants of the Sea King's psyche, but has also trapped Deadman within the King's body as part of his endgame against Blight.