LibSandstone is a modding framework and library made to make modding simpler and more powerful. It's primarily used in Reliquary, but other projects are just as free to use it.
If you are using Reliquary, you don't need to download this manually. Reliquary will handle it all for you.
All of the code is open source under the LGPLv3 license, and the Github link can be found under the Source tab.