Mask Man was an identify adopted by Mxyzptlk, a 30th century descendant of the original Mxyzptlk who fought Superman on many occasions. Apparently more murderous than his ancestor, Mask Man killed all the then members of the Legion of Super-Heroes, one after the other, except Superboy, who successfully tricked him into sayinh his name backwards - Kltpzyxm - so that he returned to his home dimension of Zrfff, and all the effects of his magic (including the deaths of the Legionnaires) was undone. The Legionnaires did not remember that they had died.