He-Man and the Masters of the Universe - Season 2 Episode 15 The Arena

Man-At-Arms has managed to make contact with a peaceful alien being called Om, hovering above Eternia. When Om is being welcomed by the people of Eternos, Skeletor invades the city with General Tataran and the Goblin Army. A fierce battle follows, and Om is confused by the way the two sides fight against one another, as his race knows no violence. So to see which side is wiser and stronger, Om selects He-Man from the side of good, and Skeletor from the side of evil, and pits them against one another in a fight to the finish.