The multiverse is dying and Earth – all of them – is the source though no one knows why.
What is known is that Incursions, a phenomenon in which two parallel Earths occupying the same super-position in the meta-structure of the multiverse collide in actuality, precedes the mutual annihilation of their respective universes. Incursions are an endless cycle. Even if you stop an Incursion more will continue to happen again and again until your Earth is destroyed.
Sheer coincidence reveals the situation to T'challa, the Black Panther, when he accidentally stumbles into an incursion point; a small bridge between the two colliding Earths where each Earth seemingly appears in the other's sky. The parallel Earth is destroyed by a group of people who came from it and Black Panther soon learns the reason. Against all previous misgivings, T’challa has no choice but to gather the brightest minds of the Marvel universe and reform the Illuminati to deal with the early death of everything.
Reed Richards explains Incursions to the group:
The situation is clear: to save universes the Illuminati must be prepared to destroy worlds. Not all heroes will accept the destruction of other worlds to save themselves so the group must operate in secrecy.
The Illuminati soon learn from Black Swan, a mysterious survivor of previous Incursions, that there are powerful beings that have successfully dealt with Incursions or accelerate the process for their own reasons. Sooner or later the Illuminati will face each of them.
Tony Stark builds his Avengers machine, assembling a group of heroes so powerful that they will be able to handle any imaginable threat without assistance from the Illuminati members as they deal with the Incursions.
Infinity
Plans rarely survive execution. Builders, a precursor race, blaze a trail of destruction across the galaxy as they head towards Earth. Their goal is simple: destroy Earth to save the universe, though explaining this to other beings is apparently beneath them. The Avengers deal with the Builders as Thanos comes to Earth for his own reasons while the Avengers are distracted.
Everyone does their job. Earth is saved from the Builders. Thanos is captured and held in stasis by the Illuminati.
The wheel is relentless and soon the Illuminati encounter the inevitable. Noble heroes from another Earth are also dealing with Incursions. The Great Society (aka The Justice League) and the Illuminati battle until Doctor Strange gives into darkness, killing them.
With no opposition left, the Illuminati debate detonating the other Earth with one of their planet busting bombs until Namor cuts all debate short.
Horrified and disgusted, the Illuminati decide amongst themselves to make peace with fate and allow the next Incursion to occur without resistance. Nothing happens. No apocalypse, no invaders, no heroes. Then the obvious: Where’s Namor?
While all of this is happening Captain America learns of the Illuminati's betrayal and receives a warning from the future.
The stage is now set. The Avengers. The Illuminati. The Cabal. Soon time runs out.
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