Michael Langdon

Michael Langdon is a minor character in American Horror Story: Murder House and possibly will become an important or even major villain in later seasons. He is the biological grandson and the adopted son of Constance Langdon, and the birth child of Tate Langdon and Vivien Harmon. He is a seemly innoncent child, but in fact he is a pure-evil Anti-Christ.

Murder House
Michael was conceived in "Pilot" when Tate, as the Rubber Man, raped Vivien, who she believed to be her husband, Ben. Billie Dean Howard believed that a child born under these circumstances would be the Anti-Christ and would bring about the end of days.

In "Birth," Vivien gives birth to him and his twin brother (named Jeffrey). He is quickly snagged by Constance. She and Moira admire him while washing off the placenta and Hayden comes in and states that he is her baby. Hayden enlists the help of Dallas and Fiona and kills Ben to prevent him from leaving with the baby and takes Michael for herself. Thanks to Travis incapacitating Hayden, Constance takes the baby from her and "adopts" him.

Unknown to anyone, Micheal is born evil. In an epilogue of "Afterbirth", after three years of the twins' birth, Constance comes home to find that Michael has murdered his nanny, revealing himself to be an Anti-Christ just as Billie Dean suspected. It is implied that Constance was both surprised and somehow pleased with that, and so that she continued to raise the child, unknowing what sort of havocs he would wreck when he grows up...

Trivia

 * Micheal is one of the most wanted candidates of fans that might return for Season 6 as an antagonist.
 * The reveal of the Anti-Christ is often considered to be one of the most surprising, the most disturbing and even the most shocking scenes ever in American Horror Story ever.
 * Micheal himself did not returned in Hotel. However, Billie Dean Howard returned in Hotel's epoligue, which set in 2022 when Micheal is seven years old. This implied that Billie Dean is yet to be Micheal's victim (if there is any possibility) even after the glimpse of the cruel fiend's nature started to appear.
 * In that epoligue, everything seems fine. Therefore, the seven-year-old Anti-Christ is yet to show its horrid malevolence completly at that time (but it is just a theory).