#undertail gaster

Happy birthday grassygalaxies@twitter :3

I have a theory that Gaster got so angry that he created the grey door out of pure spite. 

A gift for @pixie-pier!

I read your papster fanfic of priest Gaster and i just had to draw something about it uwu <3

Pairing: Grillby x W. D. Gaster (Grillster)

Featured Kinks: Sensory Deprivation, Temperature Play

Additional Warnings: Bondage

Gaster collapsed onto the stool at his friend Grillby’s bar and restaurant, exhausted after another marathon of sleepless nights working on his top secret SOUL research as the King’s Royal Scientist. On the surface, the job seemed like a dream come true for the intelligent and motivated doctor, but the dream has turned into a nightmare of failed experiments and devastating setbacks.

He needed a stiff drink. In fact, he needed about ten stiff drinks, and he was already halfway there and extremely tipsy when Grillby cut him off for the evening.

“I’m fine,” Gaster slurred when Grillby refused to refill his tumbler with more fire whiskey.

“You’re drunk,” corrected Grillby gently, “and you already looked haggard and overworked when you came in. I don’t think alcohol is going to help you.”

Gaster snorted mulishly, but he didn’t disagree. He trusted Grillby, and he knew the bartender only had his best interests at heart with the scolding.

“I just don’t know how much longer I can deal with the stress of my research,” the weary Royal Scientist admitted.

“How about I close early tonight, and we can go upstairs to my apartment and blow off some steam,” offered Grillby.

Gaster couldn’t refuse. Nothing relaxed him quite like a bedroom session with Grillby. In addition to making a five-star burger and fries, the fire elemental was a master of sensory deprivation and temperature play. Giving up control to someone else sounded perfect to a monster currently overburdened with responsibility.

Once the last patron swallowed their last bite of food, washed down by the last dregs of their drinks, Grillby flipped his business sign from OPEN to CLOSED.Gaster abandoned his sobering glass of water and sleeve of crackers to follow his friend upstairs. His head already partially cleared, Gaster now looked forward to some casual and kinky fun.

Instead of turning on the lights, Grillby lit his own way to the bedroom, leaving Gaster to stand in the darkened entrance to his apartment, awaiting his return. Grillby called out from the bedroom, instructing Gaster to strip down and leave his clothing in the entryway, and the Gaster complied without question.

The fire elemental emerged from the bedroom with a box of sensory deprivation and bondage equipment. He blindfolded and gagged the slender skeleton and placed noise blocking headphones over his skull before leading him by the hand to his living room. Grillby pushed the sofa and coffee table out of the way and guided Gaster into a kneeling position on the floor.

A spreader affixed to Gaster’s lower legs held him in locked in the position, and Grillby gently pulled his friend’s arms behind his back and tied his wrists together, attaching the skeleton monster’s bound hands to the spreader with an expert knot. Grillby’s hands itched to do some shibari knotting with Gaster at his mercy, but the tense night called for tenderness rather than the intricacies of silk rope bondage. Grillby stepped away from Gaster, allowing the suspense to build as the scientist wondered where Grillby would touch him and when.

The fire elemental lowered his flames, cooling his temperature to a pleasant warmth. Circling Gaster, he leaned forward and slid one hand up the other monster’s neck. A gust of warm moist breath tickled the skeleton’s jaw. Grillby was close enough to kiss him, holding his head in place with a firm hand on his throat, but he withdrew, leaving Gaster moaning weakly into his gag.

Grillby let several minutes tick by before he touched Gaster again. This time he increased the heat in his fingertips so that his touch would be intense but not painful. Crouching behind the skeleton, Grillby trailed his flaming fingers down Gaster’s spine, leaving the scent of warm bone hanging in the air.

The fire elemental’s warm orange glow met with competition as a glowing purple cock slowly coalesced on Gaster’s pelvis. The complementary colors cast dancing shadows across the two monsters captured in their radiance, the beauty completely lost on the blindfolded skeleton.

Removing his hands from his friend’s body, Grillby shifted himself to face Gaster, taking in the pulsing, erect length in front of him. It was still too early to let the night descend into passionate sex, but that didn’t mean Grillby couldn’t pause to appreciate the cock that the skeleton monster had summoned.

Opening his mouth wide, Grillby moved his mouth to cover the pulsating cock whose heat nearly matched his own. Gaster felt the ghostly sensation of Grillby’s mouth not quite touching him, and his cock twitched, freeing a bead of precum which dripped onto Grillby’s tongue, sizzling away to nothing in the flames.

Grillby backed away, letting Gaster’s dick bob in the cooler air that now caressed it in his absence. The fire elemental had to calm his own breathing, reminding himself not to rush. The whole point of this session was helplessness and giving up control. Libido could wait until later, though Grillby’s magic ached to form into a cock of his own.

In order to calm the fiery lust uncurling in his loins, Grillby paced. When the insistent need in his core cooled, he returned to Gaster, licking his sternum with flames that ranged from lukewarm to bordering on the unbearable. The skeleton arched his spine into the stimulation, and once more, Grillby backed away. The longer the sensory deprivation lasted, the more satisfying it would be.

The fire elemental monitored his friend closely, watching beads of sweat form on Gaster’s skull and slide down the smooth bone in ever-changing paths. Gaster’s erection subsided, the magic of his cock beginning to dissipate. Thoughts of the many tasks that awaited him tomorrow at the Laboratory crowded his mind. Grillby watched the tension- tension of a different kind- regain its hold upon his friend.

Each time his friend’s anxiety resurged, Grillby redirected his attention to some other part of his body. Grillby’s tantalizing touches both seared and soothed the skeleton scientist. Warm, crackling hands stroked Gaster’s spine, fingertips finding the spaces between the vertebrae. The magic of Gaster’s cock hardened again, engorged and throbbing from the teasing touches.

Grillby repeated the ritual again and again. Each time he aroused Gaster until the skeleton gasped and moaned into the gag, then he removed all contact and made the other monster wait. During sensory deprivation sessions, time played strange games. A single second could easily occupy an hour; an hour could pass in the blink of an eye. As time both crawled and sped by, Gaster’s reactions to the stop-and-go stimulus changed.

At first, the skeleton monster’s mind spun with thoughts of responsibilities and failures in the silence and darkness of his confines whenever Grillby stop touching him or teasing him with ever-changing temperature levels. Grillby watched his friend visibly slump each time the fatigue of the burdens of his job weighed him down once more.

Gradually, as the session progressed, a quivering anticipation replaced the defeated wilting. Gaster couldn’t focus on stress if he was seeking the heated pleasure of Grillby’s exploring hands and mouth. The inner reflections on his problems melted away into a very obvious, lust-charged wait for more of Grillby’s attention. The real world and its problems faded away under the nearly orgasmic intensity of anticipating the next sensation.

A satisfied smile spread across the dancing flames of Grillby’s features. Gaster’s surrender deserved a reward, and though the headphones blocked out the sound of Grillby unfastening his pants, the skeleton would find out soon enough what the next source of contact would be. They had both waited long enough; it was time to release the last of their stress.

Grillby left the bindings on, and it enhanced the sex exponentially. Just as the bartender intended, his skeleton friend sank into an exhausted but restorative slumber mere moments after cumming. Tomorrow would be another beleaguered day, but for tonight, the Royal Scientist rested peacefully in the fire elemental’s perfectly warm arms.

READ ON AO3