FX's new series Legion is a bizarrely beautiful, action-packed love story — that also happens to feature a lesser-known member of the X-Men. The show premiered on Wednesday, and unless you're a comic book buff, you may not have even realized that it's connected to the popular franchise. "Legion" is actually the mutant name of the main character, David Haller (Dan Stevens), who we meet in the first episode. Currently residing in a mental institution, David doesn't understand how his powers work, or even what they are. The pilot establishes that David has uncontrollable telekinetic abilities (like in Steven King's Carrie) and he can easily absorb the powers and personalities of those around him, which is why doctors, ignorant to the existence of mutants, diagnose him as schizophrenic.
So, how does Legion's story fit into the larger X-Men world? It's kind of impossible to tell Legion's story without talking about the characters we're familiar with: Legion is Professor X's son. Charles Xavier meets Gabrielle Haller while he is visiting a group of Holocaust survivors and attempting to ease their psychological pain with his telepathic powers. He has an affair with Gabrielle, she becomes pregnant, but keeps the baby a secret from his biological father. Legion and the Professor meet much later.
In the comic books, Legion's turning point comes when he accidentally murders an assassin who had killed his stepfather. He remains psychologically linked to the killer, whose personality remained in the recesses of his mind. The assassin, Jemail Karami, is one of three personas who are constantly at war in his brain; the other two are adventurer Jack Wayne and teenage girl Cyndi. He's gained telepathy, telekinesis, and pyrokinesis through these individuals, respectively. It doesn't appear that FX's series is following Legion's story to the T, but it's possible that we may eventually see shades of these characters since Legion does already exhibit these powers in the show.
As producer Bryan Singer said last year, it is "part of the X-Men universe, but when you watched it, you wouldn't have to label it — it could exist completely on its own." He noted that it "will relate to future X-Men movies." Because Legion eventually joins forces with the X-Men, it makes sense that his story will eventually intertwine with the links of Professor X, Rogue, and Magneto.