Sam's shaking more from the effort to keep himself from not moving than anything. He'd been eagerly pushing his ass up at the beginning. Now, every five slaps, Lucifer pauses, lets him catch his breath.
Sam's ass is on fire, and he's going to come before Lucifer even touches his dick.
"Ready?" Sam nods. Lucifer hovers his hand over Sam's skin to feel the heat radiating off it.
Sam cries out when he's spanked. "Thirty-six, thirty-seven"—Sam's cock throbs—"thirty-eight, thirty-nine…"
Sam collapses at forty, coming so hard he sees stars.
Lucifer rubs his back as Sam comes down. "Happy birthday, Sam."