Isaac's movements became more and more shallow, as Derek's knot began to swell inside him. Soon enough, he wasn't able to do much more than rock himself down against his Master's hips, mewling as he was filled by a seemingly never ending expansion of flesh.
"Yes Sir. Pleeease Sir!" he gasped, as Derek's teeth worked marks into his neck, the skin darkening under the rough treatment. His words trailed off into whimpers, until at last they were tied together. Derek's length pressed against parts inside of Isaac that made fireworks shoot off in front of his eyes, and to his horror, he found himself coming without permission, in hot splashes against Derek's stomach.
"Oh god, oh god... Master," he said, trying to pant out his apology.