S1·E30 The Return of Orko's Uncle

A villain called Azrog is arrested on Trolla by Uncle Montork, but he flees to Eternia where he joins forces with the witch Spydra. Montork follows him, but ends up imprisoned in Spydra's castle. When He-Man, Battle Cat, Teela and Orko go to rescue Montork, Orko falls under Azrog's spell and becomes evil, and is set against his friends.