A Fomorian king and god of the sea. In Lebor Gabala Erenn, the Milesian bard Amergin calls the waves of the sea "the cattle of Tethra" which only he can call forth. He fought the Tuatha de Danann and was killed, and now rules from Mag Mell. (Being a god, you never really die.)
He may have been a pre-Celtic god, possibly of the Picts. The Celts originally had no god of the sea, but later adopted them when they came to the Isles.