ForgivenessTale (A Original Undertale AU)

Frisk has reset the timeline again, but not before going on a Genocide run. Now that a new timeline has started, Frisk's actions come back to haunt her unmercifully every night. Nightmares have become a regular nightly companion and nothing she can do gets rid of the ghost of her past. Years after reseting, Frisk finds she can trigger a response in a monster's mind so that they remember previous timelines. A little later, she finds that after a monster learns of her actions, if they truly forgive her, it's a huge burden lifted off of her shoulders. Not just emotional burdens, but seemingly physical and psychological too. Sans and Asriel, despite the grudge they hold for one another, decide help Frisk in any way possible. Sans has an uncanny ability to "sniff" out people with the best chance of forgiving Frisk and Asriel has the strange ability to trigger "flashbacks" of the previous timeline. They are also still in the underground, but everyone has access to the ruins.

CHARACTER CREDIT TO TOBY FOX