Michael Langdon is actually conceived in the pilot of the series, once the Harmons move into their house. During one of the first nights, Ben goes downstairs in a trance. Meanwhile, the Rubber Man enters the Harmon bedroom and has sex with Vivien. In the moment, Vivien believes it's Ben she's sleeping with; they discover the rubber suit earlier in the episode, and she assumes he put it on as a joke. It's not until later that she realises it was someone else entirely.
Later, we learn it's Tate behind the mask, which plants the major seed of concern: an evil spirit has conceived with a human. And that's bad news, especially in the AHS universe.