Zaxx is an ancient four-armed being with the head of an ibis who once held the southern hemisphere of Third Earth while Mumm-Ra held the north. Two evils, one planet, neither able to finish the other. That standoff ends when Mumm-Ra tears the Rosencranz Medallion from Zaxx's neck in a battle of dark wills, and Zaxx is destroyed on the spot.
What survives is his soul, sealed in the sphere at the centre of that medallion, which Mumm-Ra then hides in his own sarcophagus so that nobody can ever let it out. Zaxx has no body after that. He has to borrow one, and the medallion is how he does it: whoever puts it on begins to lose their own life force and turn into him, four arms first.
He appears in exactly one episode, The Zaxx Factor, the third from last of the entire 1985 run, in which Vultureman steals the medallion, Snarf is very nearly consumed by it, and Zaxx and Mumm-Ra go up through the roof of the Black Pyramid locked together inside its ring.
