X-Men #11 brings back a classic mutant villain with a big secret

gamesradar.com:

Since the founding of Krakoa, the X-Men have welcomed numerous former enemies into their fold as citizens of the mutant nation, all dedicated to building a better society for themselves. 

But not every former enemy of the X-Men is keeping their chill in Krakoa - and according to the surprise ending of May 11's X-Men #11 from writer Gerry Duggan, artist Pepe Larraz, colorist Marte Gracia, and letterer Clayton Cowles, at least one of them seems to have been plotting and scheming against their own fellow Krakoans for some time.

Read on to find out who it is - though the 'how' and 'why' haven't fully been spelled out yet.

Alongside the founding of Krakoa, enemies of mutantkind responded by creating Orchis, a sinister organization dedicated to wiping out Krakoa and its citizens. One of the most instrumental figures in Orchis is the mysterious Dr. Stasis, a villainous mad scientist who is obsessed with the concept of mutant resurrection - though the secrets of how it's done have remained out of Stasis' reach.

Now, in X-Men #11, Cyclops sheds his Captain Krakoa guise to take on Dr. Stasis in his secret lair and frankly kicks his butt. Blasting a massive optic beam into Stasis' face, the villain's helmet shatters, revealing Stasis' true identity as none other than the X-Men's old foe and current questionable ally Mr. Sinister - or at least, some version of him.

Likely, Stasis is a clone of the original Mr. Sinister, as evidenced by the black club symbol on his forehead, contrasting with Mr. Sinister's usual red diamond affectation (like suits from a deck of cards). 

How and why this version came to be an anti-mutant villain remains to be seen, but it could have something to do with the hidden machinations of the core Sinister on