Michael Langdon

Michael Langdon is a minor character in American Horror Story: Murder House and possibly will become a major villain in later series. He isthe 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 an pure-evil Anti-Christ.

History
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.

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 grew up...

Trivia

 * Micheal is one of the candidates for fans that might return for Season 6.