When the edge of the dagger hits a particularly nasty weal, it's like a spark hitting gasoline. He's nothing but sensation, nothing but fuel, and he ignites in an instant. He comes in his pants, loudly, suddenly, utterly; he's been overwhelmed in the best way and suddenly all the built-up pleasure is rushing through him, inevitable and ecstatic.

He tries to hold himself still for Astarion, even through the utter white-out of his mind, by instinct and by sheer luck; it happens so abruptly he barely has time to move, except for a tiny shudder that brings up the blood just a little bit more.