Underground
Core
Just a bit farther. He thought. That determination sample he injected wouldn't last much longer, especially now that he was bleeding. It didn't have to last long, though. Just long enough. The child probably thought the determination was just an ace up Sans's sleeve. A performance enhancer. She probably thinks she won, too.
She couldn't be farther off.
As he limped through the empty metal halls, Sans collapsed. No! He thought. No more Mr. Lazy Bones. Remember what you're doin' this for. Who you're doin' this for. Papyrus. Alphys. Undyne. Tori. Asgore. Mettaton. Even that damn weed, Flowey.
He finally reached the center of the Core. The determination would fulfill it's use. Hopefully. "Doc." Sans said aloud. "I don't even know if you can hear me, but if you can, please make sure the next guys fix this. Fix everything. They can't kill the kid, but maybe they can think of something else." He swung his legs over the railing. He stared down into the burning abyss before him. "And if this doesn't work," Sans continued. "Hopefully this thing will blow that kid to kingdom come." Sans smiled weakly, closed his teary eyes, and then...
He jumped.
There was a bright flash, then nothing.
Underground
Barrier
This was it. She was so close. The erase button was an inch away from her fingertip. Then she saw a flash of light from outside. She and Flowey exchanged confused glances, then she sprinted out of the castle. She ran through the last corridor, the grey house, all the way to the Core exit.
But there was no exit. No Core. Only a crater.
"No." Chara said, horrified, and a bit impressed. "He wouldn't. He couldn't." Chara's red irises glowed bright with hatred and anger. Near endless fighting, countless resets, and he still had one last plan.
"Ch-Chara," Flowey said, quivering, sensing her rage. "We should erase this world already. We've already won."
Chara turned to the flower. "No." She said through clenched teeth. "We haven't."
"But that smiley trash bag is gone." Flowey said. "So what if he reduced himself to ashes?"
"He's not gone!" Chara yelled. The red glow filled her eyes entirely now. "Not yet."
Then she noticed something on a piece of rubble floating in the lava that once surrounded the Core.
When she realized what it was, she had an idea.
"But he'll be history soon enough." Chara said, grinning wickedly.