Avengers Mountain is built from the corpse of the Celestial, Progenitor. He had crashed landed on Earth many years ago, at the dawn of man, suffering from a cosmic infection. It was risen from its watery Arctic grave in present day by Celestials as a gift to The Avengers. They had assembled once more to fight back against the Dark Celestials, and Avengers Mountain was their reward. Black Panther, the new chairperson, had it refurbished as a new headquarters.
Major Story Arcs
As the new chairperson of the Avengers, Panther also founded a support staff, resembling an intelligence agency, called the Agents of Wakanda. One such member, Gorilla-Man, was in charge of the daily operation of the mountain, sometimes referred to as the Avengers' butler.
Unfortunately, the Avengers being based outside of the United States caused a bit of a fervor. The United States moved to institute their own rival team of mightiest heroes. They hired a newly resurrected Coulson to be in charge of a brand new Squadron Supreme of America, which was actually controlled by puppet master, Mephisto.
It also earned them the attention of Russia and Atlantis. Russia had re-instituted their own super team, The Winter Guard, covertly run by the Red Widow, who was particularly hateful of the Avengers. Meanwhile, Namor put together a team of aquatic superhumans to patrol the oceans, The Defenders of the Deep, often at odds with the Avengers over the actions of surface dwellers.
The Eternal Druig goes public with plans to exterminate mutants, believing them to be related to the Deviants. After sending assassins after The Five and the mutant stronghold on Mars, The Avengers team up with a splinter group of Eternals and their captive Mr. Sinister to resurrect the Celestial being used as Avengers Mountain to put an end to the conflict. While Captain America leads a ground team to help the mutants and contain collateral damage. Iron Man, Sinister, and the Eternals successfully get The Progenitor revived. However, this backfires when The Progenitor decides to judge the entire planet, human, Eternal, and mutant alike, forcing them to justify their continued existence.
While Progenitor continued to create tests specific to each individual on Earth and Druig continued to periodically let Uranos out of his cell to cause carnage, The Avengers did their best to keep the peace and pass whatever tests Progenitor confronts them with, sometimes secretly. Sersi brings another contingency to Tony Stark. She plans on releasing Starfox from his prison so that he could use his empathetic abilities to inspire Druig's followers into defecting and possibly even effect Progenitor itself. However, Progenitor was offended by Starfox's attempts and decides to fail the planet unilaterally.
Cap made what he thought was one last ploy to inspire the Celestial to leave the Earth be, but Cap was killed for trying. The Five resurrected him, and he settled on saving people using Eternal tech. Meanwhile, Stark led a group of mutants and Eternals into the armor of the Celestial to confront its "pilot." They couldn't fight it though, so instead they proved to it that people can not only change but judge just as harshly. The Progenitor finally relented, returning his armor back to Avengers Mountain. The Avengers reluctantly agreed to continue using it as a headquarters.
At the heart of much of the Avengers new problems was Mephisto. His centuries old manipulations, starting with a stone-age version of the Avengers made up of the first bearers of their planet's oldest heroic mantles, were finally coming to fruition.
Individually, both Namor and Jane Foster (as Valkyrie) show up to Avengers Mountain with Deathloks with messages for the Avengers. They were warning of an incoming multiversal Masters of Evil targeting Earth-616. One of the Deathloks took Robbie Reyes into the multiverse to track down more allies, while the other stayed behind, uploading his memory to Avengers Mountain. This convinced The Avengers to chase Mephisto through the timestream protecting history's mightiest heroes.
After crossing paths with the Avengers 1,000,000 BC and defeating the multiversal Masters of Evil, both teams are teleported by a sorcerer supreme variant of Loki (who had also sent the two Deathloks to warn them) to The God Quarry, a barrier at the end of the cosmos, holding back the raw energy leftover from the previous cosmos. A council of variant Mephisto and a council of variant Doctor Dooms was attacking the quarry, hoping to pierce the barrier and let the energy out. The two Avenger teams teamed up with all the allies that Robbie Reyes had collected including the Howling Commandos (an army of variant Captain Americas) and the Carol Corps (an air force of variant Captain Marvels). The Deathlok who had uploaded himself to the Celestial body was now piloting it into the fight.
After proving victorious, the Celestial headquarters was left behind in The God Quarry for the members of the Howling Commandos and Carol Corps who remained to protect the multiverse.