The lead-masked villain with the power to nullify the super-abilities of the Legion of Super-Heroes.
Superboy encounters Urthlo when he is called to the "Twenty-First Century" to aid his fellow Legion members. Because they have lost control over their powers, Saturn Girl, Lightning Lad, Sun Boy, and Cosmic Boy are given an ultimatum to leave Earth by the World-Wide Police. Just as Superboy begins his investigations, he is confronted by "a masked, flying man" wearing a purple and green costume. Because Urthlo's mask is made of lead, Superboy is unable to penetrate it with his X-ray Vision and learn the villain's identity.
Calling himself Urthlo, the stranger announces, "with this power-nullifying gadget, I can turn the super-heroes' powers on or off at will, Ha, Ha!" Keeping his reasons a secret, Urthlo promises to force the other Legion members off Earth as well and even uses "Green Kryptonite vision" to attack and weaken Superboy. However, before the Legion is totally defeated, Saturn Girl releases the heroic Mon-El from the Phantom Zone. Temporarily cured of his deadly aversion to lead by Saturn Girl, Mon-El is able to handily destroy Urthlo, who is revealed to be a robot, programmed with "hate tapes," made in the likeness of an adult Lex Luthor and sent to the future by Luthor's younger self. Defeated, Urthlo (an anagram of Luthor) vows, "my master will yet crush every last one of you!" (Adv No. 300, Sept 1962: "The Face Behind the Lead Mask!").