Of all the times Sans wished that Frisk didn't have such a fiery spirit, now was the time he prayed for it. He didn't want her to taunt whatever had stalked them. If anything, he'd wanted her to stay calm and quiet, possibly keeping her safe from this... Being.

He guessed it was another monster, but he couldn't be sure without seeing it. He'd never encountered one like this though.

Rather than being offended, the voice let out another chuckle,"Wh-wh-why don't y-youuu asssk yo-yo-your friend thereee? Sure he c-c-c-can answ-er that one." 

Focusing on the voice, Sans noted the familiar slur, but other than that, he couldn't place anything about the mystery monster. How in the hell was he supposed to know who it was? And more importantly, why was it there? What could they possibly gain from trailing around him and Frisk?

"A-a-a-alriiight then. 'spose I-I-I cannnn sh-ow-ow you," they leered.

And as the mystery figure stepped out of the smoke emitting from the crater Sans had made from his attack, the short skeleton didn't know whether to be sick or surprised. There, standing in front of him was... Himself. A distorted and negative version, but still clearly him.

His pupils disappeared as he just stared at the miscolored double of himself.

Sans was at a loss for words, but what could he say? He was facing himself. Looking at himself. Trying to protect Frisk and him from himself. So that meant the theories of alternative universes were true. No longer theories. They were realities.

The other Sans chuckled as he looked at the pair. It was really disgusting how they were holding each other like that. How they could even stand each other at all. After what that kid had done.... Why did his counterpart forgive her.

"Y-y-you two re-re-re-really are disssssgusting to watch-ch. How can yo-you even sta-an-and to be aro-und her? After all s-s-s-she's done," he commented lazily, eyes resting on Frisk with nothing but pure hatred.

No matter what universe he was in, he would always hate her.

Without another word, the corroded Sans raised his arm in the air, lazy grin still on his face, and threw it down to point at the pair. Sans watched in absolute horror as blue threads connected to the others fingers tugged down a gaster out of nowhere and hurled it towards him and Frisk. Tightening his hold on her, he dodged the blast with a roll and sent a few bones flying at the other, more so for a distraction than an actual attack. And it seemed to work as he crouched down behind a boulder and set Frisk down, making hand motions for her to stay put. 

He wouldn't let something happen to her. He couldn't.

Before she could argue, he teleported away and sent his own gaster after the imposter Sans. It almost hit him too- missed by a hair- but the other quickly countered with a barrage of bones and a chuckle. "Why-y are yo-ou-ou defend-ing her-er? We b-bo-bo-both know wha-at kindaaaa m-m-mo-monster sh-sh-she is,"he mused, shooting another gaster to punctuate his words.

Sans huffed as he dodged as best he could, a bit faster now that he wasn't hauling Frisk around, but still tired from his lack of sleep. A bone managed to barely nic his femur, making his meager 1hp quivering for a moment, but it still stood strong. As he made to dodge the gaster, he didn't see the last bone coming at him that flew between his ribs and pinned him against the cave wall.

He struggled for a few moments before finally just settling back against the wall panting,"You don't get it. Nothing comes outta hating anyone." 

The other Sans contemplated the statement for a moment and then shrugged, summoning a bone into his hand,"He-eh-eh you're p-p-probably riiight. B-b-b-but it's soooo much ea-eas-easi-easier to H A T E." 

Sans watched as the other raised the bone in his hand and aimed it for his skull.