He-Man and the Masters of the Universe Season 2 Episode 45 The Problem with Power

Skeletor tricks He-Man into thinking he has accidentally killed a man- and consumed by guilt, He-Man transforms back to Adam and drops his sword down the abyss of Grayskull, deciding he is no longer worthy of Grayskull's powers. Skeletor then opens a portal to the goblin world, enabling General Tataran's army to enter Eternia, with possibly enough power and strength to conquer the planet. Can Eternia survive without its greatest hero... and will Adam realise he has been tricked before it is too late?