That something is Warlock, who arrives at the X-Mansion (a coincidence? or not? there has been a lot of alien activity around there, maybe he picked something up) in the middle of the a party at Xavier's. Rahne, Dani and Amara have invited a couple of their local friends over, with Xavier's permission (perhaps he's trying to forestall the rising tide of prejudice, win the neighbours over before they start with the pitchforks). In a running gag, people keep assuming Amara is from Rome, New York, and expect her to act like it.
Our new character, Warlock, is a techno-organic creature, from a distant world. He feeds on lifeforce, rendering organic creatures inert masses, but can also subsist on electricity. He's unable to communicate with the humans: the breakthrough is supplied by Doug Ramsey, Kitty's friend, who has the mutant ability of hyper-communication. To involve him means outing themselves - and the school.
When Xavier gets back, they have present a new candidate for admission to the school: Warlock, who is a mutant of sorts and can now speak a kind of English. I don't know if they're actually going to admit the thing about Doug - they offer to tell the whole story to Xavier but he brushes them aside.