Sirius felt his heart hammer against his ribcage. He'd wanted this, he reminded himself. He restrained panic at the sensation of the cold metal circling his wrists. Sirius was pleased when Remus didn't slow. That's why they had a bloody safeword, and Sirius refused to use it. He wanted to push himself, to feel something once again. Pain was a break from the numb monotony of war.
Remus shoved Sirius's face down onto the bedspread, pushing Sirius's legs- shackled at the ankles- open. Sirius watched the darkness behind his blindfold as Remus pushed into him.
"Fuck me," Sirius begged.
Remus pulled the leash taut. Sirius's head snapped back, dragged as he was by the collar around his throat.
"Quiet, dog. Don't defy me," Remus answered, a frantic edge to his voice as his rhythm into Sirius didn't slow. Remus didn't slacken the leash. Sirius accepted the strain on his neck and shoulders, leaned into it. He ground himself into the bedspread, trying to find friction and release under Remus's rough attentions. The collar dug into Sirius's neck. The pain fed his arousal.
Remus seemed to know just what Sirius wanted, pushed him down face-first onto the bed again, hand seeking Sirius's erection. "Be a good pup and come for your master."
Sirius heard the wanton sound escape his lips as he came on Remus's command.
"There's a good boy," Remus panted as he released himself into Sirius's body. "Take your master's come, there's a good boy." Remus continued to rock into Sirius until he was spent.
Sirius sagged against the bedspread, and Remus released the leash, letting him.
"Merlin's balls," Sirius sighed. "That was brilliant."
"Give me half an hour and we'll do it again," Remus answered, tenderly moving sweat-soaked hair away from Sirius's temples.
"Yes, master," Sirius grinned.