Post-Pacifist but Genocide was done before

Chara exited the space ship as soon as it landed on the rocky outcropping of Mt. Ebott. Upon locating the last piece of Gaster, they had systematically taken out every monster on board. It was easier since some of them had begun to go "mysteriously missing" beforehand. Fighting Sans, however, wasn't easy even if this counted as the unofficial second fight between them.

Chara looked with cold eyes at the cave entrance. Putting Gaster back together had been a strange idea to go along with. Maybe destroying this monster once and for all would grant them a power so great the obliteration of existence would become permanent. Spending a long time in the void a monster had to become prowerful from it, right?

Chara walked forwards into the Cave of Echoes ignoring how dark it became. When the blue bioluminescence came into view, they remembered being in the Underground three years before. Letting Frisk have control was fun to watch. Seeing them befriend everyone again, and then forcing them to kill their friends later on the surface. What a joyous occasion it had been.

Chara's nostalgia became cut short by a bright flash of light. A grey door appeared at the end of the cavern full of echo flowers. They approached and opened it to a grey room where a black orb floated in its center. Taking out the piece of Gaster from their backpack and they watched the two join together.

Chara saw standing in the center of the room a skeleton dressed in black. He had Papyrus's slender and tall stature, but the smile on his face closely resembled Sans' own. Gaster looked around with drooping eyesockets little black specks falling off of his frame. When he caught sight of Chara he frowned deeply.

"you are the one who made me whole?" His voice sounded harsh from years of disuse. "I recognize the look in your eyes for I have seen it too many times. Chara."

They were pulled into the fight willingly.

"You cannot be allowed to exist."

As Chara expected of the skeleton they were assaulted with blue and orange magic laced bones. The skulls appearing out from behind Gaster were familiar having seen them used before by Sans. Chara expertly dodged the attacks and took a swipe with their knife.

Gaster managed to dodge.

"I know your sins. You must have played nice to collect all pieces of me."

Chara made it through another barrage of bones. They took another swipe, but Gaster managed to evade again. At least this skeleton was trying to be more clever than Papyrus.

"That also means my sons are dead because of you."

Chara noticed Gaster's eyes change from orange and blue to purple. An unfamiliar sensation pierced their heart as it felt like it was being tugged on by a string. Getting used to the attacks this turn proved difficult.

"Spider magic is your weakness? I suppose this time the human didn't fight Miss Muffet."

Chara gritted their teeth in frustration for a few seconds. Then, they began to smile as they brandished their knife. This fight is exactly what they needed to let this world go out with a big bang.

Gaster noticed the smile and had his Blasters keep Chara busy. He switched between the skulls and spider magic a few times to try and shake off the child's grin. Their determination was too strong to wipe away easily. He resorted to closing off the exit to the room with a black void.

"…I see now that I might be killed here. This is a room where my consciousness has spent countless hours desiring to be set free."

Gaster gestured with his hands and more of the room became devoured by the black void.

"Without my sons, my past friends, what good is getting out of here?"

Chara noticed the void underneath them and decided to test it with their knife. It felt dense like metal.

"Being trapped for eternity with you sounds pleasant compared to being free. Keeping this timeline safe is just a bonus."

The entire room became a void. Chara stared at Gaster as the skeleton stared back. He knew this game. If Gaster and Sans were really related-

"You cannot wait me out like my son. I am used to an eternity of waiting. Go ahead and test my patience."

Chara obliged with a swing of their knife. Gaster fluidly dodged to one side.

"Your determination is futile. Chara, you cannot work a miracle here with it. I control this domain and here we shall stay."

Chara narrowed their eyes and turned their attention to the floor again. They then turned around to face where the door had been. Walking over to it, Chara began to stab at where its hinges should be.

Gaster watched them stand there and stab at nothing. He crossed his arms knowing even if Chara thought they could carve out the door, the door itself was already gone. It was a concept so simple he didn't feel the need to spell it out.

After a while of pecking at nothing, Chara decided to stop. They felt the place they had been stabbing at and noticed no marks had been left. The void couldn't be penetrated from the inside.

Chara, taking stock of the situation, looked over their shoulder at Gaster. This monster had control over the room's properties. His magic dealt with more than just manipulating souls- he could manipulate space. There had to be a limit to it. It happened with Sans, and it happened to every monster.

"I see determination has yet to leave your cold eyes. You will find no limit to break here."

Chara pocketed their knife and dusted off their hands. Gaster took immediate notice and frowned. He did not move to take action.

Next, Chara sat on the floor pulling out of his pockets dusty weapons they had kept from this past year. Never knew when a monster needed a more efficient way to be killed.

"Your insult adds no injury, Chara."

They cleaned off the weapons one by one. Gaster felt himself losing his patience watching the dust scatter. Chara's actions were intentional, they wanted him to lose to anger.

After the sixth weapon, Gaster finally decided to let loose with his rage built up upon years of watching countless monsters slain by this small creature.

"I will strike you down!"

The void rumbled around them. Magic blazed in the skeleton's eyes as the very confines of space seemed to writhe around the room. Chara stood up summoning every ounce of cold blooded determination they had inside themselves. The room eventually imploded taking them both to places unimagined.

In the darkness, there appeared a light. Another light appeared and took the smaller light with it. Together, they travelled out of the dark and onto Mt. Ebott.