I had the best idea ever today
It’s not about the game before you ask, but I have something else you may be interested in near the end of this article if thats what you’re here for

I’ve decided to create a new AU, called Vitality. Read more below!

What if a handful of the main characters from Undertale had some sort of… abnormality? What if they seemed friendly on the outside, but on the inside, they held a deep, dark secret, forced to keep to themselves?

What if they were experimented on?

Long ago, Justice, the fifth human in this AU, fell into the Underground became a large threat, by killing monsters left and right. They would have committed genocide, if Asgore was not around. After Justice was killed, their soul was sent to the True Lab, where Gaster (who was still alive at the time) searched it for special strengths and weaknesses. One was found, that made the human stronger than the last four: Vitality.
Vitality was withdrawn from the soul, but Gaster had no idea if it would affect monsters differently.

Asgore ordered Gaster to perform an experiment on it, to see if it could help with protecting monsterkind. Three young subjects were taken. Given Vitality, as well as Determination, they were guaranteed to be unstoppable. Vitality would not allow them to melt, unless they were to fall down, therefore losing Vitality.

Two subjects passed the tests.
One failed.
The failure’s soul cracked under pressure from Vitality, and it changed him from curious and outgoing, to almost zombie-like, hopeless, and apathetic.
All three were still sent to train to grow even stronger when they got older.
They were sent back home when they were adults, but they were brainwashed to forget everything. Gaster died shortly afterwards, leaving Alphys to take his place. Old records were found that the Vitality could be unstable to a monster, but before word could get out, Asgore threatened to imprison Alphys if she told any of the three subjects. Now, years later, rumors are going around that another human has appeared…

I hope you’re interested! I plan to release a few pages this fall! Don’t worry, I’m still working on the game :)
Also, here’s some art I made for this AU: http://hamayuuki.deviantart.com/art/VITALITY-687548259

Now, if you have a deviantart account…
I might be hosting a contest soon, based on this game. I don’t know the theme yet, but if you want to know more info in the future, throw me a watch over there, same username as here (Hamayuuki)

Oh, did I mention that I’m making improvements to the demo? Yeah, in about a month or so, you all are gonna get a special treat! Stay tuned…

-Yuuki