He-Man and the Masters of the Universe (1983)

Season 2: Episode 46

A fierce storm, raging around Snake Mountain, uncovers a secret passage beneath the ancient fortress. Skeletor, Evil-Lyn and Beast Man investigate, and they find a mystic-looking glass called the Mirror of Moruvad, which has the power to create a double of anyone which is the opposite of what they are. Skeletor decides to capture someone close to He-Man, and make an evil double of them that will steal the secrets of Grayskull for him. He captures Koldar, a heroic ally of He-Man's, who has just arrived at the palace for a welcoming ceremony. He uses the Mirror to create an evil double of Koldar, which he sends to the Palace, to trick He-Man into letting him enter Castle Grayskull! But Koldar lures Skeletor in front of the mirror... and a heroic Skeletor is created, which goes to help foil Skeletor's plan!