The "Pariah" was Kell Mossa, a scientist who neglected his family while seeking the secrets of the Multiverse, observing them through a portal he had fashioned from antimatter. Unfortunately, his meddling attracted the attention of the Anti-Monitor, who used Mossa's portal to unleash a wave of uncontrolled antimatter that destroyed his universe and kick-started the Crisis on Infinite Earths. Mossa was saved by the Monitor, who put him to work tracking the Anti-Monitor's movements as the Pariah. He had somehow become bound to the Anti-Monitor and was forcibly teleported between realities, arriving on worlds just before the Anti-Monitor came to destroy them.
Pariah was created by Marv Wolfman and George Pérez, first appearing in Crisis on Infinite Earths #1 (1985).