Frisk had met up with Sans on his way to the Core, stopping to chat with the revenant about nothing in particular, and have a little semi-exhibitionist fun under the table at MMT resort before Frisk headed through the back exit and into the Core proper, entering the next section of the Core had a completely different feel to it from the rest of the Underground, but then again, Frisk could have said the same thing about every new location he'd been to up to that point.
Still, there was something about the Core that only spurred Frisk onward, instilling him with not only his normal lust, but a sense of energy and determination. He wasn't sure why, and he didn't think too much about it, but the feeling was there, and he let it carry him forward.
His first encounter in the Core was a two on one with the older, more powerful siblings of monsters he'd hooked up with in the RUINS. Final Froggit was much taller and stronger looking than her little sister, with spikes growing from her head and shoulders.
Whimsalot was armored, and seemed to be able to summon a weapon out of thin air with magic. Thankfully, despite their increased power, they were just as vulnerable as their siblings when it came to lust magic, and soon were succumbing to the heat.
If anything, it was easier now, as Whimsalot was less afraid of sex than her sister, and wasn't afraid to get involve with it. They pinned Frisk down, Final Froggit straddled Frisk, dropping herself hard onto his cock and grinding herself against him.
As she moaned, her long tongue extended from her mouth, pushing into Frisk's mouth and even into his throat as she rode him roughly. While she did, Whimsalot let her magical bow and arrow vanish, instead creating a new weapon to dominate Frisk with: a strap on.
It appeared on her body as she approached Frisk, already lubricated when it formed. She thrust into frisk's tiny asshole with no hesitation, grunting with the force of getting it all the way inside. Frisk moaned in delight around Final Froggit's tongue as she gagged him with it, letting the Core monsters have their way with him while he just enjoyed.
When they were done with him, Frisk continued forward, and was next confronted one on one by another core sibling to a RUINS monster, Astigmatism, older brother to Loox it seemed. He was much bigger, and his duel cock much thicker, to the point that Frisk legitimately believed that the species just grew a third leg when they reached a certain age.
Frisk gulped slightly at the sight of it, but his hesitation was nearly non-existent, and he dropped to his knees to take the long of a cock. Astigmatism groaned as Frisk took both heads into his mouth, sucking on both of them as he began to move his head slowly, sucking harder when he reached the point where the two shafts connected.
He gagged slightly when he reached the base of Astigmatism's cock, but didn't stop sucking until he felt the twin-cocked monster's member throbbing hard inside of his throat. Frisk quickly lifted his head, the tips both still in his mouth when the monster came, flooding the human's mouth, and painting his face, with white cream.
It was incredible how much the monster managed to let out; he was practically a one man-bukkakke. And once Frisk had seen that, he of course had to know how it felt inside. Astigmatism was more then happy to oblige.
He turned Frisk around and pinning him on his stomach as he forced his dual-cocks into the bitch boy's tight asshole, stretching it out hard Frisk moaned, the rough thrusting grinding his own stiff member against the cold metal floor, a bizarre level of pleasure coming from the friction against it, and a perhaps embarrassing amount of satisfaction coming when he reached his limit, cumming beneath him.
His clenching ass wasn't quite enough to finish the monster though, and Astigmatism continued to fuck frisk in a small pool of his own cum. Once that was done, Frisk found himself in one more encounter before he reached the end of the Core, another two monster team up against him, though this one was with monsters Frisk had never met.
One was female, an extremely large, even surpassing Greater dog in size. She was built like a tank, or perhaps two tanks tied together, and even when Frisk's lust magic got her to stripe out of her armor, Knight Knight's muscles alone looked like they could have blocked bullets. The other monster was much smaller, only slightly taller than Frisk himself. He looked like a magician, floating in the air with magical orb hands and no arms connecting them.
His name was Madjick, and he was just as quickly affected by the lust as Frisk. Knight Knight grabbed Frisk, pinning him down and lowering herself onto him. She didn't so much force Frisk to eat her out so much as she did use frisk's head and shoulders as a sex toy, as she was just big enough to fit the top part of his body inside of her like a living dildo.
Frisk squirmed as she did this, feeling Madjick making use of the other side of his body, thrusting into his unoccupied ass while his magical orb hands sent painful jolts of energy through his cock. The two of them, kept Frisk for awhile, trading which side they dominated until both of them were too tired to continue, Madjick running out of magical energy and freezing up, Knight Knight falling asleep against the wall.
That was Frisk needed to recover from, catching his breath before he would be ready to continue forward and face the robotic monster he knew would be waiting for him at the end of the Core.