Larian Studios Forums Divinity - Original Sin Divinity Original Sin - Mods Skill Case Study- AoE Healing Rain

It's very possible to add a custom spell to a Skillbook or anything else, such that it doesn't conflict with other stuff. Details on how to do so should be found on my Boss Summons thread, where I introduced several custom Summon skills into the game.

Essentially, a custom spell requires the following to be truly complete (if I can recall correctly)-
1) SkillData like that described in this post with a unique SKILLBOOK line at the end
2) A RootTemplate for the new Skillbook (or scroll, or potion, or anything else)
3) An entry in the Object.txt of the Generated/Data/ folder for your mod whose name is the SKILLBOOK line above
4) An entry in the Object.txt of the Generated/Links folder that combines the SKILLBOOK name with your RootTemplate GUID. Make sure the object category at the end is something like "SkillbookWater" so that merchants can stock it
5) Entries in the Mods/Modname/Localization folder that has stuff like the spells name and description. Otherwise your spell will have no name ingame. The Keys for this are based off of what you had in the SkillData

Edit: The spell I have in the first post doesn't replace the normal Rain spell, since it has a unique name and Skillbook. The same DisplayNames and such just meant that my spell would look like a Rain spell in the skillbar, it doesn't overwrite what Rain does in any way.

On the contrary, I don't think its possible to change the existing skills. At least, not without several issues popping up. Changing the Skillbooks and such so that they don't point to the normal skill is easier to do, which is what I would do if I really wanted to replace Rain ingame.