It is a lost mod from the Tmodloader, in which aspects and characters from Undertale were brought into Terraria. With this recovered mod, you can fight Sans in Terraria, and the mod was originally ported from Elements Awoken!Keep Reading Description
This mod was made for tmodloader, but was lost to time and since it's upload it hasn't received any updates. I've taken it upon myself to re-release the mod so it doesn't become lost forever as there are no signs of where it came from aside from tmodloader's mod browser. There was no original website for the mod such as a forums page and it seems that the mod doesn't have a known author. As such there is no way to track down it's original creator to find out if the project was cancelled but it can be assumed that it has been.
I do not own Terraria, Undertale, and their respective properties.
Simply extract the manifest(.json) file and put it in your mods folder(under documents, where you find your maps for terraria).
I won't be able to update the file for newer versions as this was just a recovery of a lost mod for the community.