Michael Langdon is the son of Tate Langdon and Vivien Harmon.
Murder House takes place in a uniquely macabre Victorian mansion in Los Angeles. The Harmon family moves there from Boston after their patriarch, Ben, had an affair, and his wife, Vivien, a miscarriage. In the very first episode of American Horror Story, Vivien has sex with a man in a black rubber suit she believes to be Ben, but who we later learn is Tate Langdon in disguise.
Tate Langdon is the ghost of a teenager who in 1994 murdered a handful of his classmates in a school shooting, before a SWAT team killed him in his home, the titular Murder House. His spirit is now trapped in the house with several others, whom Tate befriends. One of them is Nora Montgomery, whose baby son was murdered in the 1920s, and who later committed suicide in the house. Tate promises Nora he'll give her a baby, so he rapes Vivien. The conception produces twins.
Vivien, unaware that she's carrying the children of a ghost, has a rough pregnancy. Ben eventually has her committed to a psych ward, believing her anxiety about the house to be a mental disturbance. She returns to the house and dies giving birth to the twins, one of whom is a stillborn. The other is Michael Langdon.