Warning: SPOILERS for The Flash season 6, episode 8.
The last episode of The Flash leading into Crisis on Infinite Earths confirmed the theory that dimensional explorer and self-described mythbuster Nash Wells would become the Arrowverse's version of Pariah. While it had been known for some time that actor Tom Cavanagh would be playing the pivotal Crisis character, it was unclear if the latest incarnation of Harrison Wells to appear on The Flash would become a servant of the Monitor or not.
In the original Crisis of Infinite Earth comic book miniseries, Pariah was a scientist who neglected his family while seeking the secrets of the universe, observing them through a portal he had fashioned from anti-matter. Unfortunately, Pariah's meddling attracted the attention of the Anti-Monitor, who used Pariah's portal to unleash a wave of uncontrolled anti-matter that destroyed Pariah's universe and kick-started the Crisis. Pariah was saved by the Monitor, who put him to work tracking the Anti-Monitor's movements. Pariah 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.
Given that most versions of Harrison Wells from around the Arrowverse were brilliant scientists who neglected their families, it made sense for a version of Harrison Wells to become Pariah. However, the question remained as to just how this would happen. An answer was seemingly provided by a teaser trailer which aired after the final episodes of Batwoman, Supergirl and Arrow leading into the Crisis event, which showed Nash Wells pledging himself to the Monitor's cause. This raised a new question; how did the cynical Nash Wells, who had come to Earth-1 to kill the Monitor, wind up swearing fealty to him? It turns out, Nash Wells believed the Monitor saved him from a pack of Bloodwork's zombies, and so, he pledged his loyalty to his savior.
The answer to why Nash Wells had a change of heart was revealed in "The Last Temptation of Barry Allen, Part 2"; the final episode of The Flash heading into Crisis. Early in the episode, Wells stood before the door to the Monitor's lair which he had discovered, loudly declaring that he had come for the Monitor. For the first time the Monitor responded, saying that he knew Wells sought knowledge and that "if you wish to know all, show your allegiance and all will be revealed." Wells refused to humble himself and bow to a being he considered a false god. However, before he could make good on his threat to storm the Monitor's lair, Wells was attacked by a group of Blood Brothers, the zombie-like minions of the villain Bloodwork.
Just before Wells was overrun by the Blood Brothers, an energy wave washed over them, freeing them from Bloodwork's control. Wells took this as a miracle; proof that the Monitor had saved him to prove his good intentions and show himself worthy of devotion. In truth, the cleansing energy was the work of The Flash and his friends at STAR Labs. There is some irony that Wells, who devoted his life to exposing fake gods and false prophets should find his faith as a result of a lucky coincidence. Then again, with the power at the Monitor's disposal, could it have been more than coincidence?