2x27 - He-Man and the Masters of the Universe

When Orko finds out that Adam's visiting cousin, Prince Dal, doesn't think Eternos is as magical as his own home kingdom, he gives Prince Dal a pretty rock and tells him that its called the Star Crystal and that it will protect anyone who has it. Prince Dal believes the lie and steals the rock from Orko later to bring to his father, who is fighting a war against the Torks, thinking it will make him invincible.