Aquaman hunts for Mera in Adventure 476 (Oct 80), in a story by J.M. DeMatteis and Dick Giordano.
He runs into a powerful man who claims to be the god Poseidon, although it’s fairly clear he is not. When he explains his past he is clearly an amnesiac survivor of a shipwreck who comes across a mysterious but powerful trident, the source of his powers.
He has nothing to do with Mera’s disappearance either, and there is little significant about this tale, aside from the gorgeous art.
That, and the fact that their battle leads them into the town of New Atlantis, not seen for decades in Adventure, but re-introduced in his World’s Finest run.
Oswin hold the powerful staff of Mn’torr, and thus the upper hand, as the Starman story opens, by Paul Levitz, Steve Ditko and Romeo Tanghal.
While Starman tries to hold off Oswin, Jediah attempts to get Clryssa and Merria to safety, but they refuse to leave. Jediah follows the Empress’ orders and departs, hooking up with the freed convicts as a sort of resistance movement, while Oswin defeats and captures Gavyn, Clryssa and Merria.
Oswin imprisons Starman in a crystal chamber seemingly impervious to his powers, but Gavyn manages to escape, and he and the evil Lord Protector face off for the last time. The resulting explosion appears to kill them both, but Starman leaves behind a message indicating that he survived.