Dan Stevens — formerly and famously of “Downton Abbey” — is returning to TV, in the FX “X-Men” pilot, “Legion.” It hasn’t gone to a full series order, but with Stevens — also Jean Smart, Aubrey Plaza and Rachel Keller, one of the breakout stars from “Fargo” 2 — onboard, that’s just a mere formality.

The pilot is written by the very busy Noah Hawley (of “Fargo”) with assists by “X-Men” impresarios Lauren Shuler Donner, Bryan Singer, and Marvel universe mega-expert Jeph Loeb.

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Per FX, which just announced, Stevens plays the lead, David Haller, described this way: “Diagnosed as schizophrenic at a young age, David is 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.”

“Legion” isn’t exactly an “X-Men” adaptation but is drawn from the expansive “X-Men” universe: Haller is the son of “X-Men” founder Charles Xavier, aka Professor X, played by Patrick Stewart in “Days of Future Past” (and the next Wolverine movie).

Stevens is a newcomer to the X-Men universe . . . and Marvel one too. His Matthew Crawley left “Downton” — tragically — at the end of the third season. “Downton” fans have since recovered — barely. A huge fan favorite, Stevens had a busy career awaiting him and the open arms of Hollywood. As a return to TV, “Legion” is about as splashy and gaudy as it gets.