"Ungghh," Isaac gave a rough, inhuman noise, as his own eyes glazed over into wolfish Beta-yellow. It wasn't like him to shift during the middle of sex, but the knowledge of what he was about to do overwhelmed him. He was going to take his Master's knot, something that had haunted his fantasies since long before Derek had ever touched him in a sexual manner.
The muscles in his legs were already straining, as Isaac started to work himself up and down on Derek's length. With each stoke, he dropped himself down hard, impaling himself. He could smell his own blood in the room, as the marks left by Derek's claws struggled against the wolfsbane to close up.