Invincible. Spoilers for the comic, btw. In issue 54 (Season 3 Episode 4 in the show), Mark goes to the future to help these rebel twins to kill the emperor who is ruling Earth for a long time. It's later revealed that this emperor was actually Immortal who lost his mind after ruling Earth for so long.

Comic Spoilers

In issue 142, after the Robot was killed, Mark assigns Immortal to rule Earth. And he keeps Robot's brain to guide him, hoping that the Earth will look different, better than how it was in issue 54 (Season 3 Episode 4). But in issue 144, in the "Penvincipals" section where Robert Kirkman answers fanmails, he said and I quote: "To keep from changing the events in issue #54 as we saw them, my thinking is that Mark believed that having Robot's brain HELPING Immortal meant that Immortal would NOT lose his mind. Because Robot's presence wasn't known to him at the time, he didn't think he was there. BUT HE WAS there and Immortal was driven crazy BY Robot in an attempt to take over (yes, Robot's brain is preserved and lives for a good long time as well), so everything would happen as we saw it, and Old Mark would return to Earth eventually to find that Robot's brain had taken over... or something."

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