Author's Notes
Just a short prologue to kick this story off before I actually get down to business with it. It's been a very long time since I've written anything, so pardon me if the quality isn't so good.
Artist credit for the cover art goes to kawaii-chibi-kotou on deviantArt [NSFW Warning]
It was finally over. Frisk's adventure had finally come to an end, the barrier was broken and the monsters were free. It was a brand new day for the denizens of Mt. Ebott and there was much work to be done to ensure a peaceful coexistence between humanity and monsterkind.
While many monsters elected to venture out into the world, many also remained at home, in their cozy mountain. Most were just happy to have the freedom to leave, even if they had no intention of abandoning their old homes. Most of those who left built humble settlements at the base of the mountain, slowly but surely expanding outwards from there.
King Asgore worked on building human-monster relations with the help of Papyrus, who had taken it upon himself to act as ambassador, and Undyne, his ever-present right hand and one of the few who were capable of keeping Papyrus in check. She and Alphys now called the surface world home, living together in a mid-sized house built alongside many others at the base of the mountain. Alphys decided to leave her old lab and the terrible memories associated with it behind, partly because she wished to put that chapter of her life behind her and partly because there was no way Undyne could live with her in the Hotlands.
Sans and his brother, however, remained in Snowdin. The town seemed much emptier at this point, most of the residents having moved on to the surface world. Grillby was still around though, and that was all Sans really needed. All things said and done, he felt pretty good.
As for Frisk, after everything they had been through, they just wanted to settle into a cozy home with Toriel, who graciously accepted them with open arms and an open heart. After all, there wasn't anything back in the human world for them. They returned with Toriel to her old home in the Ruins, just a short walk away from Snowdin.
But Frisk wasn't happy. Frisk hadn't been happy for a long time now. Unbeknownst to everyone else, this wasn't the first time Frisk had made this journey. In another time, another place, they hadn't been as kindhearted. Distant memories flooded their mind on a daily basis. The phantom sensation of being covered in a dusty substance, the familiar feeling of a tight grip on the hilt of a blade, and the surge of adrenaline that came with cutting down monster after monster. It haunted them, racked them with guilt.
They weren't sure what drove them to take the path that they did. At first, they were just scared and lonely. Everywhere they turned, something was trying to kill them, but the more they fought back, the more they found themselves enjoying it. It escalated first to who was merely in the way, until finally it became a game to hunt down everybody.
And then there was that spectre. 'Chara', it called itself. Every step of the way, Frisk screamed at themselves to stop what they were doing. Things didn't have to be this way. If only they'd listened sooner. Once they were past the point of no return, the only way to go back and make things right was to give up their deepest possession.
Their SOUL.
"That wasn't so hard, now was it?" Chara had told them. "You're a great partner."