Uncanny X-Men (Uncanny X-Men Vol. 6 #13)

“THE DARK ARTERY” Starts Here! In their most terrifying adventure yet, the four young OUTLIERS discover the true meaning of Haven House and a doorway that should never be opened. Meanwhile, an otherworldly presence has set its covetous eye on one of the four young mutants, in a story that tells a never-before-told slice of the history of mutantkind!