Who is Legion’s Yellow-Eyed Demon? The FX series, adapted by Fargo’s Noah Hawley, is currently airing its third and final season. Legion centers on a man named David Haller, the son of Charles Xavier and one of the most powerful mutants on the planet. He’s also a diagnosed schizophrenic though his abilities, such as telepathy and telekinesis, are largely to blame for his instability.
Another major contributing factor to David’s deteriorating mental state is the mysterious Yellow-Eyed Demon. This creature had been plaguing David’s thoughts since the show’s pilot, making him - and the audience - constantly question what’s real. Though Legion is loosely based on characters from the comics, it’s this villain’s involvement that ties the show to the larger X-Men universe. The Big Bad's reveal also finally shed some light on their father/son relationship.
So, who exactly is Legion's Yellow-Eyed Demon? He’s appeared as everything from a horrifying monster to an adorable puppy to Aubrey Plaza’s Lenny. His name is Amahl Farouk and in the comics, he’s better known as the Shadow King. The revelation that he was the one wreaking havoc in the mutant's mind also cemented the identity of David's father as Charles Xavier. After battling Charles on the astral plane, Farouk wound up trapped there. In his quest for vengeance, the villain had infected David's mind since infancy. Keeping David ignorant of his presence, always appearing as anything other than himself - even a persona as thinly veiled as a dog named King - the Shadow King had been siphoning David’s powers for years.
Legion's Shadow King shares a fair amount in common with the character's comic origins. Amahl Farouk was introduced in Uncanny X-Men #117 by the legendary team of Chris Claremont and John Byrne. He was the latest human host to the Shadow King, who had existed for centuries. Farouk and Charles met in 1970s Cairo and it was really his first experience with a mutant so intent on using his powers for evil. In fact, it was this meeting that inspired Charles to form the X-Men. In the comics, Farouk hadn’t possessed David since he was an infant but had taken control of him in an effort to destroy Muir Island and kill Charles. Professor X emerged victorious, though David was left comatose for quite some time afterward.
Legion is an incredibly unique take on the superhero show, largely eschewing typical tropes of the genre. Bringing in the Shadow King was really the first time the series embraced its comic lore. The villain has played a major role in the series ever since and his relationship with David has evolved quite a bit over time, particularly now that David has become the villain of his own story.