After years of clever teases, Legion has finally introduced its version of Charles Xavier aka Professor X, leader of the X-Men and David Haller's biological father. David's telepathic powers have grown in strength to the point he could likely rival his dear old dad, but David's been using his powers for decidedly darker purposes. In the current third and final season, Division 3 is still attempting to capture him not only because he's destined to end the world, but because he has yet to answer for his crimes against his former girlfriend, fellow mutant Syd Barrett. As their relationship began to fracture, David altered Syd's memory so she would forget their problems; David even slept with her afterward, the sort of violation so heinous that it served as the definitive moment where David became the villain of his own story.
How could the child of Professor X stray so far from the light? Well, a lot of it is due to Xavier himself. We're introduced to Legion's version of the iconic telepath - played by veteran actor Harry Lloyd - in a post-World War II psychiatric ward; Charles had some horrible experiences during the war that surely scarred him, but he mostly resided at the hospital to help others with his mutant abilities. He eventually meets Gabrielle Haller, a young woman whose experiences in a Nazi concentration camp left her essentially catatonic. Charles slowly nursed her back to health, and the two eventually fell in love. When he deemed Gabrielle well enough to leave, he telepathically altered the doctors' opinions about the both of them and left to start their new life together.
Gabrielle - who was still incredibly fragile and haunted by the war - soon gave birth to a baby boy, David. When David was an infant, Xavier used an early version of Cerebro to find his first fellow mutant, and left David and Gabrielle to look for him in Morocco. The mutant, of course, would turn out to be Amahl Farouk, also known as the villainous Shadow King. After Xavier and Farouk had a mighty battle on the astral plane, Xavier emerged victorious, seemingly ending the Shadow King's reign of terror. However, Farouk's consciousness survived, and attached itself to baby David as a sort of psychic parasite.
It's still unclear exactly why David was given up for adoption, but the pieces are starting to come together. In the earliest days of the show, we learned that David was a deeply troubled child and teenager, constantly in trouble with the law and in-and-out of mental health facilities. Gabrielle was likely too unwell herself to raise a child literally possessed by a demon. In the comics, Xavier never knew he had fathered a child with Gabrielle until he met the adult David, but in this version Xavier not only knew of David, but raised him for at least a little while. Why he would give up David remains the show's most compelling question.
One of the show's great ironies is that David ended up contributing to his own disownment. Present day David and mutant time traveler Switch went back in an attempt to prevent the Shadow King from possessing him, but the only thing they managed to accomplish was exacerbating Gabrielle's issues by seeming like random whispers and far away music to her. The exploration of mental illness has always been at the heart of Legion, and the fact that David negatively affected his own mother's wellbeing in an attempt to help himself and, perhaps, inadvertently solidified his fate in the process is one of the sadder beats the show has explored. As Legion begins its endgame, it seems unlikely anyone short of Professor X himself will be able to stop David. That would be an appropriate sendoff, since a lot of this mess can be laid at the feet of Xavier's hubris, believing himself to be the ultimate authority, even when that self-importance serves as a detriment to his family, and possibly the catalyst for the end of all things.