Hope someone else can answer definitively, but if this is the first 616 mention, then that effectively establishes it as a thing in this continuity well enough for me
Either way this (and Nimrod's mention of "incursions") raises the last big story written by Hickman, i.e. the Secret Wars death of everyone and everything - that makes me uncomfortable because how could Moira's death not have triggered her rebirth in each of her 9 lives so far?
The clones are also a Gillen thing. And the camp..because it's fun? At least it certainly was in Gillen's hands! I think he actually did elaborate on his reasoning at some point, but it may have been on a now defunct social media platform he used to have (what was that? oh, Formspring, I think!)
I think the phrasing is mostly figurative (although because it's comics it's also almost literal).. it was demons, of Limbo. First S'ym and later, primarily, N'astirh.
Is Inferno worth catching up on? (Who were the main writers?)
I haven't read Adamantium Agenda so maybe that will fill in the missing piece of my understanding of this clue, but others pointed me towards Spider-Man and The X-Men #6 from 2015 which has Ernst reveal that she's been acting as a mole for Sinister in exchange for him giving No-Girl a body. But she ultimately fights against him alongside the X-Men and her betrayal only gets her 5 months detention. I was wondering how/why this could be the thing where Hickman needed to specify that the samples were switched, though, because in the issue I just read, Sinister's lab is destroyed but it's not like they specify that all his records were destroyed. Does Adamantium Agenda have something like that?
Anyway, to answer your main question, that very issue of Spider-Man and the X-Men has this panel I pulled that confirms that as of 2015 at least Ernst was still around and the Cassandra Nova thing still seems in-play...
Attachment 86959
Umm, it's been a little while since I read Adamantium Agenda, but it certainly ended (spoilers... alert?) with our heroes destroying Sinister's stash of all mutant DNA -- which Hickman is now suggesting were swapped out beforehand. (It also established that someone in Xavier's Institute is not really a mutant.)
This leads me on a different tangential thought -- Apoc and his suicide squad in X^2 must have obtained more information from their raid of the machine databases than merely the date Nimrod came online (you wouldn't need a massive hunk of crystal "injected" into Mother Moira to record one date) -- maybe it's sinister's mutant DNA log?
Not sure what it means (fashion sense runs in the family?) except that the outfit, and the fact that Hickman's research apparently may have included the comic panel I just posted, makes it seem that Cassandra is not far from Hickman's thoughts...
Cassandra is certainly formidable -- I've got Morrison's run but I can't recall if he was the one who retconned her from being Charles's killed-in-the-womb twin to an alien psychic parasite. (I have also said that I hated the way he revealed Xorn as drug-addicted, deathcamp-building Magneto... who only lasted a few issues before getting decapitated again, after killing Jean again -- what a clusterfuck of an ending to a series with such powerful concepts!)
I can't recall the X-Men encountering Surtur. There were those 80's annuals where the X-Men and New Mutants visited Asgard.. but no Surtur IIRC. Bringing in that mythology would be almost like a meta nod to how Walt Simonson's Thor overlapped somewhat with his wife's time as the main X-book writer besides Claremont. The Mutant Massacre crossed over with Thor too... but not in a "mixing of mythologies" type way. I feel like the closest we've got to that is Dani becoming a Valkyrie...
Yeah nah, I've got the X Men and New Mutants in Asgard annuals with OroThor and Dani Valkyrie -- fun stuff, but no Surtur...
Skipped your questions I didn't really have anything for! (not even a rambling side comment, lol.)