Arkon the Magnificent first showed up in Avengers #75, but his cover debut was the King Kong swipe cover for Avengers #76 above.
Arkon was the ruler of the planet Poelemachus which was suffering an energy crisis. It was figured out that by setting off atomic explosions on the dimensionally identically located Earth that the energy of Polemachus could be recharged.
To this end Arkon, a barbarian in the Marvel Universe proper travels to Earth by means of a spell cast by Wanda Maximoff, the Scarlet Witch and kidnaps some scientists to get them to build a bomb to do the deed. Quicksilver gets the Avengers involved to save Wanda and the battle is on.
After that threat is averted, the Avengers end up on Polemachus in Avengers #84 to rescue the Black Knight when he is taken captive by Arkon and his ally the Enchantress. After that Arkon and Polemachus fade out of Marvel mythology for a while.
Arkon shows up years later in an X-Men Annual but I've always mostly been fascinated by his early Avengers appearances.
Arkon seems to me to be another Roy Thomas precursor to Conan. Despite his sci-fi origins, as rendered masterfully by John Buscema and Tom Palmer, he looks every bit the part of a savage barbarian, very like a certain Hyborian not yet a part of the Marvel pagentry. Plus Arkon had some cool sci-fi elements, the other-dimensional stuff and his quiver of lightning bolts, which do different things.
If anything, Arkon's notorious bad temper and epic arrogance make him a great villain, though he's always shown with a edge of concern for his people's welfare, if not also his own. Maybe because we saw relatively little of Arkon, and because he's so fundamentally unlikable as a person, he has ever been a fascinating figure in the greater Marvel Universe.
Here is a gallery of original artwork from the pertinent Avengers issues featuring Arkon.