Sans wakes up, sweating, his bones shaking with magic and his soul aching. "so it begins," he mutters, groaning as he rolls over in bed. Already his head is throbbing. He needs, he needs... you. But he isn't selfish enough to go straight to you. Besides, he'd just... scare you off with his intensity. He drags a hand down his face and gently ruts into the pillow next to him, groaning. He hates doing this, but at least it's something. Plus, Papyrus should be gone for work already, so he's alone. Quite unfortunate that you work on Saturdays...
He gets up and quickly lifts his shirt off, straddling the pillow. His soul lights up his ribcage, and it is secreting a viscous substance. He reaches inside slowly and pulls out the glowing heart of essence, sighing in satisfaction as his phalanges rub against it. A slightly less intense glow begins to emanate from below him, and he knows his cock has materialized, as well as his tongue.
With a huff, he sinks his fingers into his soul with one hand and begins to desperately rut into the pillow, squeezing it with his other hand. He throws his head back and a string blush suffuses his face as he whines in pleasure, pelvis slamming against cotton, cock throbbing. He imagines, instead of the pliable plush of the pillow, your smooth skin, your wet heat, the way you say his name so reverently. Another glow appears, from behind his eyelids, and he knows this is the part where his magic will get out of control.
His magic gathers in the air in several spots, glowing blue, and materializing into disembodied hands. They float for a moment, flexing their fingers, then swoop in towards him. Simultaneously, magic erupts from his ribcage, taking on long, coiling, solid shapes, tendrils oozing from his clavicle and sternum and out from the bottom of his ribcage. One disembodied hand is stroking his spine, one has its fingers tangled in his ribcage, and another is furiously pumping his cock. The tendrils wiggle and wrap around his soul and snake into his skull through his mouth. A few more wrap around his legs, in between his tibia and fibula.
All the while, sans is panting, tongue hanging out as he frantically ruts against the cushion. He knows he won't ever be satisfied, unless he has you, but he also knows it would be unbearable to ignore his heat. His whole body is almost completely surrounded by magic, every bone stimulated (ba-dum tsss) and he becomes completely immersed in his own pleasure. So immersed, unfortunately, that he doesn't realize that he's been at it for literally hours.
His heat is intense enough for him to lose track of time, and immersive enough that he doesn't hear you coming home early.
(A/N): i fixed the formatting lmao i didn't even realize
also this is probably the nastiest thing ive ever written pls kill me