I'm not caught up on the show but early in the second season, it was made pretty clear that the rings would give Sauron control over flesh. So if he has finished making the One Ring where you are at in the show, I would assume that is what happened. He is able to control them via the ring.

As a side note, my favorite moment of the second season was when they show a mom orc carrying her baby orc as though we're supposed to feel a measure of sympathy for the orcs because they can have children or something.