Modding

Rain World mods come in a few varieties. The most user friendly approach is to use the official level editor to create new rooms which can then be imported into the engine with relative ease. It's also possible to use the tool dnSpy to peek at the games base code and potentially make modifications or create entirely new creatures or mechanics, but given the complexity of the codebase it is extraordinarily challenging (even for an experienced C# programmer). Finally, there are a few community made tools (i.e. Unofficial Level Editor by AndrewFM and Mikronaut) that give you access to some of the information and allow for the placement of enemies or the tweaking of pre-existing engine variables.