Mon-El was a Daxamite put in the Phantom Zone a thousand years earlier because he was sick from lead poisoning. Saturn Girl detected his presence and he warned the Legion of a serious threat. Brainiac 5 freed him from the Zone and created a temporary cure for Mon-El's lead poisoning. He delivered a super-powerful bomb (actually a Phantom Zone projector) to the Dominion's core to defeat the Dominators (sending them all into the Phantom Zone), but his lead poisoning returned and Brainiac 5 had to place him in a safe place until he could create a permanent cure.
