The intrepid PCs intervene moments too late to stop the cultists' ritual. A gate to elsewhere opens. But things don't turn out exactly as the evil Professor Simonton, the heroes' recurring nemesis, planned. Instead of contacting the Plain of Leng or the Nameless City or Unknown Kadath, the cosmic portal leads to ... Eternia. Yeah, that Eternia. Suddenly Skeletor and his minions are loose in 1920s Boston (or wherever your campaign is set), and it is up to the player-characters to stop him. He-Man's stuck in his own dimension, puzzled but relieved to get a break from the Big Bad's endless schemes.
Skeletor's got high-tech gadgetry, a sorceress, and mutant musclemen on his side. (So far, par for the course for a typical CoC encounter.) His energy weapons and vehicles will eventually run out of juice and ammo, and he'll have difficulty compensating with Twenties Earth tech. As usual, his ambition exceeds his competence and that of his goons, who may have their own agendas. On the other hand, there are surely mouldering tomes and ancient artifacts in the adventurers' world that could prove useful to him.
Could it work? Magical super villains might make an interesting break from the usual Mythos opponents. On the other hand, CoC focuses on suspense and mystery, and subtlety isn't Skeletor's strong suite. Try keeping this one out of the papers, guys.