The Skeletal Hero: Karmic Judge

True ending

Flowey gets a soul.

Sans confronts Gaster and Chara before bringing them to MHA.

Gaster's sludgy shit turns into a normal body, Chara has to work with Gaster to make Chara a soul.

____

Monsters were free, Sans was free, so why didn't it feel like the end? 

Everyone is happy, even Flowey- now Asriel again is somewhat happy, if a little confused about what to do next, but still. The story should be finished, so what is he missing?

...

Sans knew.

But he didn't know what he was going to do. He didn't know if he could do it.

But it needed to be done.

It wasn't fair.

And Sans wouldn't be able to go forward with a clear enough conscious, knowing he's ignoring the last that needed to be saved. 

Sans pushed himself off the couch and walked down the hall, he leaned in the doorway of the room, knocking on the rim. The other man glanced up at the boy with a raised eyebrow. He sighed.

"You have that look on your face like you're about to do something stupid." Aizawa muttered, Sans, let out a short chuckle.

"well, trouble is my middle name." Sans smirked, Aizawa rolled his eyes.

"I thought it was comic." The man huffed. "So what shit have you gotten yourself into?" He asked, the boys smile tightened.

"it's a surprise, but i just wanted to let you know i might be gone for a little while." Aizawa grimaced, Sans always spoke like that when he was about to do something reckless, and it always set Aizawa off. "so- bye." Sans stopped the man from releasing the questions about to barge, shortcutting out of the house. 
He walked down the street and into a park, wandering to a secluded spot.
"alright, sans. just gonna free a hatred-filled soul and a mother fucker, nothing we can't handle!" Sans lied to himself in a bad attempt at convincing himself this was a good idea. "it has to be done."
The skeleton summoned his soul out of his chest, hands hovering around the red heart as determination-powered magic swelled around his hands. His hands twitched in a semi-familiar pattern as he shaped out the portal. He brought his middle finger and thumb together. 

*click*

The red circle opened up into a void of darkness, Sans stepped through. 
The floor felt odd beneath his feet, like black jelly on the verge of melting, yet refused to give way to the weight pressed against its surface.
Just across the way sat a child wearing a green shirt with a singular stripe across it along with a man devoid of any colour.
The younger of the two saw the newcomer first, perking up slightly. Chara stared at him for a moment in confusion.

"About damn time comedian! We've been waiting for ages!" Chara's shout echoed around the barren darkness.

"i had a lot to do, at least i came!" Sans shouted back, walking over to the two. Gaster was staying silent, Sans could basically see the guilt radiating off of him.

"What the hell were you doing anyway?" Chara asked.

"freeing monster kind." Sans answered nonchalantly, both Chara and Gaster stared at him, shocked.

"Already?!" Chara shouted in surprise.

"yep, hardest part of it was finding out how to get there, but the answer was handed to me on a silver platter." Sans shrugged.

"[...How is everyone? Are they okay?]" Gaster glanced at the boy briefly.

"yeah, everyone's adjusting well, they're really enjoying it." Sans answered. "which reminds me." He paused, looking between the two seriously. "i want you guys to come back with me."