The Cleric was an alien missionary. His faith lead him to Krypton, where his peaceful message and denunciation of cloning quickly gained him a following, with Syra and Sen-M among them.
His teachings didn't liked to the Kryptonian Science Council, wh belived the Cleric was threatening Krypton's way of life and developed the Eradicator to kill him. It merely incapacitated him, but it instigated a riot that left many dead. Cleric took the Eradicator and left the planet, taking a large portion of his followers with him. Sen-M stayed behind to lead the League of Life.
However, the Kryptonians needed a link to their planet, and without it, they slowly withered away and died. Cleric felt responsible and went into exile. What he did not know was that the Eradicator had become linked to him and prolonged his life. He seek refuge in an asteroid orbiting Warworld, where he eventually meet and friend Cellkeeper 385, who keep him hide to Mongul.
Some 200,000 years later, the Cleric meet the last survivor of Krypton: Kal-El, as known as Superman, by then self-exiled from Earth and trapped in Mongul gladiator games. The two exchanged memories of their darkest moments, and Cleric decided to give the Eradicator to Superman. Doing so lifted the device's protection and Cleric rapidly aged and died.