"Not all stories need a pacifist. This story is one of them". - Chara, at the end of the True Pacifist Run.
The True Pacifist route is where Frisk spares everyone. However, unlike Undertale, True Pacifist route is considered to be the bad ending.
How it all goes down Edit
The True Pacifist route is when Frisk disobeys Flowey's orders to stop the monsters, and become friends instead. It's exactly like Undertale's True Pacifist route based on the events that occur, but when they reach New Home, it becomes similar to the final act of the Genocide Route.
When Frisk reaches Judgment Hall, Sans tells them to kill him. Frisk refuses, and Sans forces them to do so, leading to a fight that's similar to the Sans boss fight in Undertale. However, when Frisk chooses to spare, Sans starts to cry and hugs Frisk, telling them that they are "the only good friend that they'll ever have", and let's them proceed to Asgore's room. If Frisk chooses to fight, Sans kills them on the spot, mocking on how manipulative they are.
Upon entering the throne room, Asgore prepares to fight, but all of the other monsters stop him from doing so. This makes Flowey snap, and he captures the monsters in vines, planning to do the job himself. Suddenly, Frisk hugs them, and convinces Flowey to be a kind spirit. He let's them go, and proceeds to help free the monsters....by sacrificing their soul to save them.
Frisk ends up in the void, and meets Chara, "the Angel that calls by their own name". Chara tells Frisk that by sacrificing their soul, they freed the monsters and let them reign havoc on the humans. Chara gives them a chance to make a better choice by resetting the world. If they choose reset, the game does a soft reset, and Frisk starts with a new save file. If they don't, Chara takes their soul and does the reset themselves.