Late in X-factor's run there were some people from Bishop's time added to the group for reasons I can't remember and didn't really care about at the time either. (X-Factor got really bad towards the end once they decided to add Shard, Sabretooth, and Mystique to the team.) One of the characters was trying to travel back to his own time and caused an explosion with a time machine that Havok got caught up in.
Meanwhile at that exact same moment in an alternate reality a different Havok was killed by a sentinel blast.
Our Havok then woke up in that Havok's body. He tried to convince people he was from an alternate reality for a while and for the most part nobody listened. Eventually he decided to just stat living his life as that Havok, who was a well respected member of one of THE Superhuman team on his earth. He was also married to Madelyn Pryor and had a kid.
Eventually, Maddie goes nuts again and becomes Goblin Queen except this Goblin Queen is a cosmic entity. Beyonder gets involved merges with the Goblin Force and Havok has to stop them. He discovers he somehow has ties to the Nexus of All Realities (that thing Man-Thing is always guarding.) After his battle with Beyonder Goblin Queen the next time we see him is back in our reality where his unconscious body is being taken care of by everyone's favorite supporting cast member Nurse Annie Gxyzptlk. This was the Chuck Austin era of X-men so his time in the Mutant X world and the Nexus was ignored.
I think there was one story in Exiles that touched up on it though.
Mutant X was a fun little title for the most part where every single character was living their own little What if? Iceman never gained control over his powers again after Loki messed with them. Scott Summers was taken into space and became a Starjammer. Storm was bitten by Dracula and became a vampire. Spider-man had six arms. Captain America was a mutant (and apparently this wasn't the same Cap from WWII), instea dof giving Archangel metal wings Apocalypse made him more demon-like. Dr. Doom was a hero. Stuff like that.
Mutant X, and the somewhat contemporaneous Earth X(and it's spinoffs), represent some of the deepest lore/continuity based comics I can think of, and I loved them! Those were comic book readers comics. Say what you will about their complexity, but in many ways that sort of thing is awesome compared to the stripped down, decompressed, rootless, floating in nothing comic books that are more in vogue these days. *I will say Hickman's new era is a perplexing blend of both continuity heavy and continuity-less storytelling. I love stories built on top of other stories.
Let the flames destroy all but that which is pure and true!
Eh, I didn’t know about Storm’s Dracula incident or Iceman’s deal with Loki, and I still had a lot of fun with it. Mutant X was the O5+ with a monstrous twist. It was fun little book that really shouldn’t have been more than a 12-issue book. Alas, it had enough fans that writers decided it needed to address the Phoenix.
Earth X is majestic in its scope. I think it does work to clear the Byzantine, actually. Celestial antibodies should be the foundation the Marvel Universe is built.
Is Avengers 1Mill BC even in continuity?
That feels like some weird What if thing. At best it seems like it'll later show up as a comic or TV show I character is checking out only to have them say how silly the whole thing is.