This is the modloader that allows mods to function.
You need this to install mods into the game.
To install simply drop everything from the zip into your game folder.
steamapps/common/Monster Sanctuary
After this is installed any mods you download will go into Monster Sanctuary/BepInEx/plugins
Config files will be found in Monster Sanctuary/BepInEx/config

Sources:
https://github.com/BepInEx/BepInEx
https://github.com/MonoMod/MonoMod
https://github.com/BepInEx/BepInEx.MonoMod.Loader
https://github.com/EvaisaGiac/MonSancAPI