One of the biggest mysteries of Jonathan's Hickman's X-Men saga has been resolved. In the X of Swords Handbook, the section on Moira MacTaggert confirms exactly how she survived the Legacy Virus and went on to continue living in secret. How did Moira survive, and why did she apparently fake her own death when she's apparently always had the ability to reincarnate?
In Hickman's House of X, one of the biggest reveals (and major retcons) was the revelation that Moira MacTaggert was not simply a gifted human geneticist and past love interest of Charles Xavier, as she had been believed to be for years. Instead, Hickman retconned her origins to have her be a mutant with the ability to reincarnate into a new timeline after death, awaking in utero with all of her memories and experiences intact, to live the same life again and again... unless she chooses to change it.
Moira's power allows her to live multiple lives, with each life seeing her tackling the concept of her own power and the existence of mutants in general. In one life she found a cure for the X-gene, while in another she abandoned the cure after seeing what the terrible results were. She also strove to prevent the future of mutant annihilation by Sentinels, and in other lives she rejected Xavier, then united with Xavier, then fell in love with Xavier. With each and every life she tried different tactics to make the world better, especially for mutants. However, this retcon led to some confusion as to how - in stories preceding Hickman's run - she died from the Legacy Virus as the first human to become infected. In reality, fans now know that it was because she was a mutant all along.
However, regardless of her being human or mutant, it has now been confirmed in the X Of Swords Handbook that Moira faked her death from the Legacy Virus, and that the Moira that died was instead a Shi'ar golem that believed it was the real Moira (basically a clone.) This provides the explanation for why Moira was seen twice in the afterlife - during Dr. Strange's battle with the Pitiful One and during the conflict with the Chaos King - when she was in fact secretly alive on Krakoa the whole time.
The reason for faking her death is also revealed. For her tenth and current life, Moira determined to live in secret, preparing the foundation for the eventual mutant nation of Krakoa, clandestinely working with Xavier and Magneto to make the dream of a peaceful mutant existence a reality. Apparently the secrecy was a key component so as not to to compromise any part of Krakoa's formation into a brand new mutant society, or her current plans to aid mutantkind in general.
What's really interesting here is not just that Moira faked her death, but the specific note the handbook makes that it was the golem who was in the afterlife, and not the real Moira. The golem itself was apparently an early experiment of Cerebro's capabilities, which is what was used to copy Moira's memories and implant them within the golem, making it believe that it was truly Moira herself. The golem was the precursor to the resurrection protocols Xavier now has in place on Krakoa to bring back fallen mutants using clones and backups of their memories stored within Cerebro. This does beg the question: if Moira's golem had enough of a soul to be present in the afterlife, what happens when and if the real Moira dies? Will there then be two Moira MacTaggerts in the afterlife? Is the same true for any other fallen X-Men and their clones? It's certainly worth thinking about in a world where fallen heroes are so often called back to the land of the living.