Notes:

Sauce: http://nutastic.tumblr.com/post/131378860456/me-likey-ricksancheezes-undertale-au-underfell

For those who want to know w-- the Underfell AU is, I'll spell out as much as I can by copy pasting the answer someone else did.

"You know how all the monsters in Undertale are essentially good in nature, despite their flaws, and Flowey is evil? Well, Underfell is a design experiment in making Flowey the only GOOD creature down in the Underground. Frisk discovers Flowey is their only ally against a world of monsters seeking revenge on humanity rather in addition to freedom, and killing the monsters to keep them from escaping and destroying humanity seems to be the only option.

A more recent twist on the idea, though, is that befriending the monsters–the option that seems least plausible and is most difficult–can also work.Frisk would show the monsters that humanity isn’t all bad and that they really shouldn’t seek revenge when peace is a better option.

In this route, much like how you only fight Sans in Undertale’s “Genocide” route, you’d only fight him in Underfell’s “Pacifist” route because he is the most reluctant to change his ways."