At the end of chapter 5, Kris, Susie, and Ralsei have seemingly cornered the Knight and the final battle begins. In the background is a large, flowing fountain that will soon start the Roaring if it is not sealed. At the end of the fight, it looks like the Knight is going to be defeated, until he slashes Kris, separating them from You. You fall into the fountain, and Kris is left soulless and weak as the Knight escapes. The Roaring is beginning.
In the beginning of chapter 6, You wake up in the depths, and since you’re not controlling anyone, the Undertale fight system is used. At first, you seem to be in a Happy Town. However, this mirage is broken when the security robot, the Trouble Dingle, chases you out of it. You lose the Dingle, but it continues hunting you down throughout the Meat Factory, where all of the vessels are made. Later, you supposedly escape the Factory, but the Dingle finds and corners you. Right as it’s about to kill you, dialogue appears on screen “STAND DOWN”. The Dingle backs away, and the camera pans up to reveal…
W.D. Gaster.
“I KNOW THIS ALL MUST BE A LARGE, LARGE UNDERTAKING FOR YOU, ESPECIALLY THIS VERY MOMENT, BUT NOT FOR ME. NOT FOR ME. EVERYTHING TO THIS POINT HAS BEEN CALCULATED BY ME THOUSANDS, THOUSANDS, THOUSANDS OF TIMES. EVERYTHING HAS BEEN MONITORED BY ME FOR THOUSANDS, THOUSANDS, THOUSANDS OF HOURS. THOUGH, I AM VERY, VERY PLEASED TO MEET YOU, [name that you wrote in the beginning of the game]. I, TOO, WANT THE BEST FOR KRIS, FOR EVERYONE, BUT EVERYTHING THAT HAS OCCURRED HAD TO, AND YOU MUST ACCEPT THAT. I DID NOT WISH TO SPEAK TO KRIS HERE, THEY HAVE NO REASON TO BE HERE, BUT YOU DO. YOU ARE MY BRAVE VOLUNTEER, YOU HAVE ALWAYS PARTICIPATED IN MY SURVEY PROGRAM, AND YOU HAVE BEEN VERY, VERY WONDERFUL IN DOING THIS EXPERIMENT WITH ME. BEFORE, MY WORDS WERE YOU CANNOT CHOOSE WHO YOU ARE IN THIS WORLD, BUT IT’S TIME TO TWIST THOSE WORDS. I SAID YOU CANNOT CHOOSE WHO YOU ARE, BUT YOU NEVER DID. YOUR VESSEL WAS A REFLECTION OF YOURSELF, CORRECT? YOU DID NOT CHOOSE YOURSELF, YOU CHOSE HOW TO EXPRESS YOURSELF. IT’S TIME TO STAND AGAINST FATE AND OWN YOUR VESSEL.”
After this, you must fight the vessel you created, who is the final boss of the chapter. If you do the weird route, this boss is exponentially more difficult. Regardless of the route, you fuse with your vessel while some variation of Your Power plays. “SPLENDID, SPLENDID. EVERYTHING IS GOING ACCORDING TO MY PLAN, NOT FATE’S. THE MACHINE THERE, IT WILL TRANSPORT YOU TO THE BUNKER, MY DISGUISE FOR THIS PLACE. STEP INSIDE. OH, AND THERE’S SOMEONE ELSE I WANT YOU TO MEET. COME OUT, MISS. YOUR PRESENCE IS REQUESTED.” Dess enters the room. “Your crazy plan worked, huh.” “CORRECT. NOW, JOIN [name] ON THEIR MISSION. I’VE KEPT YOU HERE LONG ENOUGH.” “Y-you’re kidding, right? I- I can-“ “YES, YES. WHO AM I TO KEEP YOU HERE FOR ANY LONGER?” Dess enters the machine alongside You. “Thank you.” “I’M SORRY… ANYHOW, [name], TAKE THIS.” It’s a pneumatic tube containing a light blue soul. “RETURN THIS TO KRIS AT ONCE.”
Gaster approaches the control panel for the machine and powers it on. Before activating it, he says “REMEMBER, YOU TWO, EVERYTHING AFTER THIS HAS NOT BEEN CALCULATED, NOR PREDETERMINED BY FATE. IT IS UP TO YOU: TWO VARIABLES FATE OVERLOOKED. IF YOU FAIL, IT WILL BE YOUR FAULT.”
Gaster activates the machine, and the chapter ends.
Chapter 7 begins in the bunker, which is a mix between Asgore’s living space in Deltarune and the True Lab. You can learn about Gaster and Dess’ history there via scientific logs and Dess’ dialogue. At one point during the chapter, there would be a Kris mini boss because they think you’re trying to take control of them again. The fight ends with You giving Kris’ soul back. During the final battle with Fate, You and Kris would be hit by a large blast, thrusting their soul out of their body and You out of your body. The two of you fuse one last time for the final final final battle.