Invincible's evil variants of Mark Graysons are abruptly shunted out of the episode before the climax, setting up their potential return.

It's worth noting that this didn't come entirely out of nowhere though. Angstrom Levy, the genius who kickstarted this war, appeared all the way back in the first episode of season 2. And like in the comics, his storyline, the build-up to The Invincible War, actually developed over a long period of time.