Krakoa perfectly demonstrates the X-Men's ability to be a unifying force for the mutant population of the Marvel Universe, uniting plenty of former enemies under the dream of a grand future for the mutant race. It wasn't the first, however, with one such possibility coming with an impossibly high price.
What was Planet-X from Uncanny Avengers, and what did this dark future have in common with the current mutant nation of Krakoa?
Two of the greatest enemies the Uncanny Avengers ever encountered were Uriel and Eimin, the children of the corrupted Dark Angel. Taken in by the time-traveling Kang, the pair carried out a series of machinations to destroy humanity and elevate mutant-kind. To that end, they killed a Celestial, dooming the world to the wrath of the Celestial Executioner, Exitar. The mutants of the world were teleported aboard an interstellar ark along with Eimin -- Uriel falling to Thor before the end of the conflict -- by utilizing the powers of the Scarlet Witch. Ensuring that the Avenger was then promptly killed by Rogue, Eimin hid her actions and escaped along with the rest of the mutants.
The Earth -- and everyone on it -- was promptly destroyed by the Celestial, and Eimin led the survivors to create Planet-X. Within six years, society had reestablished itself with a hierarchy -- with Eimin in control alongside the appointed X-Council: Cyclops, Jean Grey, Storm, Psylocke, Quicksilver, and Cable. The history of this world was tweaked by Eimin, placing the blame for Earth's destruction on the Avengers -- including Havok, who at the time was one of the only surviving members of the Uncanny Avengers team. He was forced into hiding alongside Beast and one of the last surviving humans, the Wasp. Havok and Wasp even get married and eventually have a child together. Eimin set into place a powerful mutant government and helped transform their new world into a technological paradise.
Serving under Eimin was Magneto, the leader of a renovated X-Force. Magneto recruited the Brotherhood of Evil Mutants to work alongside him as a peacekeeping force. The remaining Avengers were forced to work with Kang and a collection of reality-displaced heroes and villains to strike back. Meanwhile, Eimin is able to convince her allies on the Council to bring down Havok -- while planning to then have her horseman Daken assassinate them in order to cement her power. While Thor and Kang's warriors attack Eimin -- freeing a captured Wolverine and Sunfire -- Havok is able to convince Cyclops to trust him. This leads the X-Council to turn against X-Force. Using Kang's technology, the Uncanny Avengers go back in time and save the world, altering the future and erasing Planet-X from existence.
Planet X is, in many ways, surprisingly similar to the current incarnation of Krakoa. The island nation is somewhat separate from the rest of the Marvel Universe, although some heroes have made a point of maintaining contact and relations with their human allies. Planet-X by contrast was a world built by survivors, who've come to dislike the memory of the human race even more so than in most realities. It's even ruled by a council, similar to the current Quiet Council. However, old divisions still exist in this mutant world, allowing for more obvious schemes and hatred against each other. It also wasn't as united as Krakoa, allowing figures like Magneto to lash out instead of tempering his fury as he has in the modern era.
In many ways, Planet-X feels like a possible way that Krakoa could have been flawed from the start, with even a fresh coat of paint and a new world failing to truly unite the mutants. It's an indicator of what Krakoa could be like if it wasn't for the collective dream at the heart of the nation.