What Type of Mod?

Hello, I am wanting to put together a mod which adds some items and possibly skills and enchantments into the game, and I would like to be able to use it with custom games as well, however, I don’t know if there is one particular type of mod which would allow this. Any ideas? Thanks in advance.

Mods, as opposed to tools like Grim Internals, can only be used as custom games. They can’t be used in the vanilla game.

I suppose that makes sense, what about using it with custom games? Would I have to merge them?

No, your mod would effectively be a custom game so you’d have to choose it under Custom Game to be able to play it.