Underpray is a more religious AU, in which monsters were sealed in a forest. In this AU, our good friend Sans is a Guardsman, with a permanently glowing eye that is shaped like a targeting scope. His weapon of choice is a Gaster Blaster Bow that fires blue energy arrows. He is slightly more depressed than Undertale Sans, but still tells jokes. In other interpretations of the AU, Gaster!Sans takes the place of Underpray Sans. Underpray Gaster!Sans has facial markings all over his face, and instead of using a bow, he uses a scythe-like weapon.
To see more of Underpray, check out this video: ☀https://www.youtube.com/watch?v=JZA8IQ_go3o
I, (Felki), admittedly do not know much of Underpray, but I'll do my best!
In this AU, Sans is one of the royal guards of the <<Danger Zone>>. Frisk is sent into the <<Danger Zone>> on a mission to watch what's going on and make sure that nothing is wrong with the monsters living their. In this AU, Frisk is a girl. Just keep that fact in your head, seeing as this AU is probably a Frans shipping AU.
Frisk is given a pentagram, which will take away all her powers of resetting if she speaks. Because of this, she is taught sign language and goes through other intense training before heading into the <<Danger Zone>>. Apparently, Frisk was supposed to destroy something with her powers, and later, after that was done, the pentagram would take her powers away anyway. (Just some random facts that you may or may not know.)
Sans was (supposedly) hired by the humans to make sure that Frisk had a safe journey through the Danger Zone. Once Frisk leaves Toriel, she meets Sans and interacts with him.
In this AU, Sans also smokes.
Later, when reaching the Lab with Frisk, they are confronted by Gaster, who orders Sans to kill the child. Sans promptly refuses.
Later, when Undyne has orders to kill Frisk, she and Sans pretend to fight while actually trying to destroy Alphys's camera. (According to the video by SuccubreRukia, but her stuff isn't exactly cannon.)
In this AU, Sans's main weapon is his bow and arrow.
Another weapon that Sans uses is his Sythe.
(Unfortunately no picture for that.)
Some more information about Underpray Sans:
"•Sans is, in a sense, part of security as a Guardsman.
•There were no Guardsmen when the first Dancer arrived, but because the first Dancer died, Guardsmen were put in place.
•Sans doesn't have a good feeling about humans so he doesn’t rescue any when a human is about to die.
◦But, for Papyrus, he’s always around humans that arrive.
•This Sans has the same personality as the original Undertale Sans.
•He treats humans and Dancers differently.
◦He is on high alert when around humans, but feels a bit of pity and shame for Dancers.
•He blames himself for not having been able to stop Gaster.
◦Therefore, everytime he sees her (**Frisk?? Dancers? Not stated exactly), he blames himself and ends up loathing mankind even more, but he is really confused by this behavior of his.
•He despises and hates mankind, but doesn’t show it."
What's a dancer? If you know, please feel free to edit and share.
(More Underpray stuff can be found here: ☀http://1startwinkle.deviantart.com/art/Underpray-AU-Translation-Incomplete-606353454)