In the comics, he’s the reason The Monitor puts the worlds’ heroes to the test, in hopes his significant challenge will prep them for the Anti-Monitor’s big plan.
There is one character we’ve yet to see in the lead to “Crisis On Infinite Earths,” and that’s Anti-Monitor.
We’ve yet to learn if the crossover will introduce Anti-Monitor, or if he’ll play a factor, although it wouldn’t be surprising to learn that if they did, he’d appear on The Flash first.