Spoilers for Invincible below!
Invincible has had a few comic crossovers that span multiple franchises, but the wildest crossover was when Robert Kirkman's superhero found himself in a zombie apocalypse akin to Kirkman's other popular franchise The Walking Dead.
Kirkman is the author of both popular comic franchises with Invincible being his (along with artists Cory Walker and Ryan Ottley) take on superheroes if they acted like real people with believable foibles and flaws. A similar strain of narrative realism runs through The Walking Dead. Invincible tarries in more fantastical style and scale with Mark Grayson venturing into space and fighting alien conquerors. But before all that Mark fought a foe that could open up portals to alternate dimensions.
In one of the more tragic plotlines of Invincible, Mark is confronted by the villainous Angstrom Levy when he kidnaps and assaults Mark’s mom in Invincible #32. He sends Mark through various dimensions of Kirkman’s own creation and through a few others that have similar spandex from other superhero comics.
Mark is seen tackling Doc Octopus (much to Spider-Man’s chagrin) in the Marvel Comics dimension. Only to be hurtled into a universe littered with zombies. This likely isn’t literally The Walking Dead universe. These zombies repeat the word “meat” over and over, a characteristic that none of The Walking Dead zombies have ever exhibited as they only groan or hiss. Kirkman has toyed with expanded multiverses of his other properties but this one appears to be a brand new zombified universe filled with a lot more campiness than the grim and sober Walking Dead. Robert Kirkman has done this before, utilizing his own characters in a multiverse.
Mark is ported into multiple other dimensions fighting off deadlier and deadlier foes as Angstrom tries to weaken Mark to make it easier to defeat him. In one dimension Mark is seated around a campfire with what appear to be hardened survivors of a zombie apocalypse. That could be The Walking Dead universe, given that Rick Grimes already has an appearance in Invincible’s home universe, but there's no signature zombies or black-and-white tone. Image, unlike Marvel or DC Comics, doesn’t play as much with multiverse-hopping heroes where all alternate realities merge or diverge based on a larger narrative arc. There are separate dimensions with distinct realities not often meddled with by other writers. Certainly, there are character crossovers in Invincible, like Spawn and Savage Dragon. But those characters have their own stories to follow unrelated to Invincible. Kirkman may dabble in his own multiverse with his own properties, but it usually doesn't change any narratives in other franchises.
The Walking Dead never featured a brightly colored half-alien flying around pulverizing zombies in its own pages as it would have been completely out of place both in tone and style. But Mark appears to have entered a similar dimension to The Walking Dead, just unbeknown to the central characters of either franchise. It’s an easy out to forgo altering The Walking Dead storyline and makes for a fun easter egg in Invincible. Instead of altering multiple universes and juggling different versions of the same character, Invincible prefers to have one cataclysmic storyline where dimensions cross. Invincible makes his first “kill” with Angstrom. Only for the portal-creating villain to regain his health and put together an assassin squad composed of all the evil versions of Mark across multiple dimensions and sets them loose on the primary Invincible universe. Multiple dimensions were never a problem The Walking Dead dealt with since zombies and unhinged survivors are just as dangerous as any superhero villain.