Mordru the Merciless was the tyrannical ruler of the planet Zerox. The Legion fought him and found his weakness, resulting in his capture and imprisonment in an airless chamber deep within the Legion HQ.
Mordru’s freedom came when Shadow Lass, busy flirting with Mon-El, opened the vault and allowed air to enter, reviving him. His power coruscating freely, Mon-El, Shadow Lass, Superboy and Duo Damsel were forced to flee back to Superboy’s time, where they disguised themselves, and hypnotised themselves into believing themselves ordinary to escape his vengeance.
Tracking them down despite this, Mordru captured them – together with Pete Ross and Lana Lang - and magically assembled a court to decide their fate. Pete Ross’s defence broke the spell holding the prosecutor, Wraithor, an old enemy of Mordru’s, and he helped the Legionnaires escape their tomb before dying at Mordru’s hands in doing so. Superboy and Mon-El caused the cavern they were in to collapse, trapping Mordru again.
Mordru later escaped to trouble the Legion repeatedly over the years. He took over the medieval planet, Avalon, where he was defeated by Star Boy. He was also the power behind the Earthwar.
