Dire Bare
Legend
Legion is a part of the X-Men universe before Marvel/Disney reacquired the rights . . . so is not part of the MCU (Marvel Cinematic Universe). So, Shadow King has not yet appeared in the MCU and is a possibility. But I agree, Legion was an awesome and trippy show, and did a fantastic job with Shadow King . . . I also hope he's not involved in WandaVision. Does Shadow King play a role in the comics as an antagonist to the Scarlet Witch and/or Vision?I hope not. Only because of how much I enjoyed how Shadow King was brought to the screen in Legion.
For those who aren't aware of this gem, Legion is a three-season TV series in the X-Men universe about the powerful mutant David Haller, otherwise known as Legion in the comics. Haller has mental abilities on par with Professor X and other psychic mutants. It was an FX network show, and can be streamed on Hulu. It has gotten mixed reviews, and can be a bit hard to follow . . . but is a lot of fun. Actors involved you might recognize include Aubrey Plaza and Jermaine Clement.