Jeremie Harris has joined the growing cast of "Legion," FX's new "X-Men" universe-based television series. He will play Ptonomy, who -- according to The Hollywood Reporter -- is "an outsider by nature, stand-offish and sardonic who is a former child prodigy who feels trapped in the past."

Dan Stevens will star in the series as David Haller, described as "a haunted man, trying to find his way back to sanity, but he's getting tired and is about to give up when he meets the girl of his dreams." Stevens and Harris will also appear alongside Aubrey Plaza, Rachel Keller and Jean Smart.

FX's X-Men-Based "Legion" Enlists Dan Stevens, Aubrey Plaza & Jean Smart

Harris' past credits include a recurring role on Netflix's "The Get Down" and guest spots on "Bessie," "Blue Bloods" and "Person of Interest."

"Legion" is in development at cable network FX, with "Fargo's" Noah Hawley on board as a producer. According to the official description, "Since he was a teenager, David has struggled with mental illness. Diagnosed as schizophrenic, David has been in and out of psychiatric hospitals for years. But after a strange encounter with a fellow patient, he's confronted with the possibility that the voices he hears and the visions he sees might be real." Production on the "Legion" pilot will begin in March.