Gillen definitely wasn't kidding about this event having ramifications in the current timeline! It's an oversized issue with a lot of details so I'll summarize things as best as possible.
We open with the preview pages, the undead Galactus outside the Brotherhood's palace. Moira is outside the vault where the lab with the Moira clones is held and she can't get in as the Arakkii won't allow her. Sinister is standing over the bodies of Jon Ironfire and members of the Arakkii Five, communicating with Moira. She asks him to try to change Jon's mind about letting them into the lab and Sinister says he changed Jon's mind by pushing it out of his skull when he shot him (see SatB #3). They're running out of time as he assumes Beast is on his way now that Emma has been taken out. He tells Moira that Destiny's message alluded to having a way inside to the lab and he waits, hoping something is about to happen. Moira suddenly says the Marauder, his ship, has picked something up traveling towards them. She beams a holographic image to Sinister of what the ship has spotted and he instantly recognizes the Juggernaut and remembers how they shot him at Thanos a millenium ago. He deduces right away that Destiny had planned this from way back then.
Juggernaut flies through the head of the Galactus and straight into the lab vault, busting it open with a massive explosion. Moira says she's going in but Sinister says to wait for him as he has to upload his plans. Before he can meet her though a kunai-rope dart/harpoon impales him just below the shoulder. Ironfire drags Sinister to him and says he coated his bones with his metal blood thus surviving the shot to the head. Sinister yells for Moira not to go inside and reset the timeline as we see her walking inside.
We cut to a Red Diamond spaceship and Beast is inside sitting on Emma's throne, flanked by Esme and two other Cuckoos, with "Namornauts" standing around as guards. He's aware that the lab is exposed now and wants to send the Namors to retrieve the Moira engine but says they need a distraction. He says it's time for an alliance and orders the Cuckoos to dismantle the Quiet Council psychic barricades. The Cuckoos warn him of the risk of connecting with someone but he insists.
We cut to Xavier in what I like to call his think tank as he's out somewhere on the frontlines of the "Enforced Serenity War". Charles is surprised to suddenly hear Beast in his head, guessing Beast must be panicked if he's dropping the psi-shields. Charles reads his mind to see what Beast wants and when he learns of the Moira clones, he astrally projects into Beast's ship and fully takes everything/everyone over, taking the lead in the fight. "To me, my dreamers", he says. "Fight for your dream! Worry not! The dream is an idea..." CORRECTION: Xavier uses the minds of his followers to take over a planet of the World Farm.
Sick! Ironfire and Sinister reel back from the initial psychic backlash. Jon says with the Brotherhood's psychic defenses gone and all their psychics dead, they're in trouble. Sinister says he himself is not in trouble because he has his own defenses, Jon is the one in trouble. He says he once had an Omega psychic that could have protected Jon but she's dead. Jon is like well, since I'm the leader of the Brotherhood I might as well look the part, and creates for himself a Magneto helmet with his blood to protect him from Xavier. Smart! Sinister begs Jon not to kill him and to instead turn his attention on Xavier and that he can help Jon stop Xavier. He pleads with Jon, telling him it's better to let him go try to reset the universe than to allow Xavier to gain control of the Moiras, turning this hellscape universe into an even worse one. He offers to inject Jon with Bobby Drake DNA as the chilled water will supply Jon with all the blood he needs to become even more powerful. Jon agrees and let's him go after Sinister injects him, reminding Sinister that he will kill him at some point.
Jon is now massive and is flying towards the Xavier controlled purple headed ship. Xavier says the metal of the helmet is blocking him. He starts to recognize the metal, not having seen it in a very long time.
Xavier: Erik?
Ironfire: Nah. But you should know...
Oof! Sinister enters the lab and shoots his little Doom gun at Moira, telling her to get away from his Moira's. They start arguing, Moira saying he turned her into a machine and he's like girl look at you, you turned yourself into a machine. By now she has killed Mother Righteous and Banshee (see Nightcrawlers #3). They argue some more and she shoots at him, trying to prevent him from uploading his files because she doesn't want him learning from any of this after they reset. He says he must upload the info because if he doesn't learn from it it'll all just repeat. He says the more time they waste the more likely Xavier will come take control of everything, as we cut to Xavier starting to overwhelm Ironfire outside and the Namornauts speeding towards the lab. Moira begrudgingly agrees and says he better demolish the Moira engine the second he returns to the main timeline. Sinister uploads his files while she's fighting off the Namors and but then he hesitates, wondering if he should take everything he has learned from this universe and truly change or... "maybe I can become a god."
Data page describing Sinister's Inferno Fail-Safe.
He's going for Dominion! While she's still fighting the Namors, Sinister tells Moira about his psychic inferno fail-safe which can be initiated right there from the lab. "It will kill every single person in the universe with my modified X-Gene. And use them as fuel." He tells her not to look so horrified, what's one more atrocity when they were going to wipe the dimension away anyway. He presses a button and we get a series of panels as the sinisterized mutants across the universe die. The Namors, Xavier and his followers, Exodus and all the clones that feed him. Interestingly we see Wagnerine surrounded by Nightkin skulls being absorbed but she seems to be protected by a forcefield as she's holding her baby. Shaw and everyone in the Waiting Room hell die too. Sinister is absorbing all of them and he's glowing red, flying higher and higher as he absorbs more and more. "I soar on wings with untold trillions of bony feathers." He's about to leave time itself, becoming all times at once... then he bumps into a giant red wall. The space is blocked, he can't get through and ascend higher. A Dominion is already there!
Sinister: What are you?
An unknown voice: I am the being that is harvesting the power you stole, as whatever is yours is mine now. I am a being of infinite knowledge and perspective. I could tell you everything. Instead, I share a single fact. Who am I? Not you.
Sinister is back in the lab, on the floor. "No! I'm too late. A Sinister Dominion is already outside time and space. It happened in another timeline earlier... I'm too late. It's not me." He looks devastated, and then Jon Ironfire walks in.
Jon: You killed a galaxy.
Sinister (looking defeated): I did. For nothing. One of them does it. Stasis. Mother. Orbis. Not me. Kill her. Kill me. Kill yourself. Nothing matters anymore.
Jon: You're wrong.