You were doing a genocide route and you and your sadistic killing spree already had got you to LV 19, you kept walking, until you eventually entered a long hall, with a lot of pillars and windows, you didn't lose time and saved your progress in the yellow star checkpoint, after all, anything could happen

You kept walking and walking until you came across a figure on the other side of the hall...

"heya."

When you took a good look at the figure, there you saw, Sans The Skeleton, the lazy smiley skeleton whos brother is dead by your hands =)

"you've been busy, huh?

Sans stares at you in silence for a moment before resuming

"so, i've got a question for ya."

Sans closes his eyes

"do you think even the worst person can change...? that everyone can be a better version of themselves, if they try hard enough?"

You take a step in Sans' direction

"heh heh heh heh.. all right. well, here's a better question kiddo."

Sans opens his eyes, and they were completely black empty

"do you wanna have a bad time? because if you take a step closer..."

Sans chuckles a bit

"you're REALLY not going to like what happens next."

You take another step in Sans' direction and he closes his eyes

"welp. sorry, old lady. this is why i never make promises."

FIGHT START

birdsong could be heard in the background

"it's a beautiful day outside, birds are singing, flowers are blooming... on days like these... kids like you..."

Everything goes black for a moment, and when everything is back to normal, Sans had completely black eyes again

"Should be burning in hell."

You are then attacked by a quick pattern of down throw, maneuver through wavy bone path, and a Gaster Blaster pattern

The attack was very sudden and intense, so it caught you off guard, getting you to 50HP, but you still have healing items left!

"huh. always wondered why people never use their strongest attack first."

Sans said with a slightly amused expression

YOUR TURN! (oh also Megalovania started playing, man this song...)