Who's This? The disco version of Dr. Strange on page 19 of Who's Who vol. XVIII.
How you could have heard of him: Both Mark Merlin and Prince Ra-Man have appeared since, but it's not clear how their continuity was ever resolved. Prince Ra-Man is usually seen as a dead or erased figure, most notably in Grant Morrison's work (Animal Man's Crisis II and in the guise of King Ra-Man, an entity encountered by Zatanna in 7 Soldiers), and as the leader of the rebellion in Purgatory in the Reign in Hell event. An older, retired Mark Merlin was featured in Detective Comics, but then Sword of Atlantis featured his widow Elsa and their cat, only for Merlin to appear alive and well again in some Superman comics asking the younger Zatara to help him find Prince Ra-Man. So convoluted is still the order of the day.
Example story: House of Secrets #74 (1965) by Bob Haney and Bernard Baily
While all this angst is going on, at one of Cloister's downtown skyscrapers (Cloister is in Vermont according to Mayfair's Atlas of the DCU, by the way, just north of Montpellier), a man panics and tries to throw himself out a 70th-story window, claiming people are after him. It's weird so Prince Ra-Man visits him in the hospital.
From what I've seen, comics had a lot of trouble keeping to supernatural themes in its comics, probably due to the Comics Code's stringent criteria, but also because SF and superheroics were on the rise. In Bob Haney's mind blender, they're all the same, so the villain uses both without really mixing them. It's odd. As Mark Merlin, he did face alien invasions and such as well, so it wasn't new to Prince Ra-Man. And it's likely why this type of feature failed in the end.
Who else? Well here's the thing. Normally I'd be switching to volume XIX, but lately, I've been thinking about ramping this feature into high gear, which would necessitate some changes in its mission. Look for a discussion on this possibility later this week.