Scripts

How can I change the icon and description of a spell, instead of summoning Ilm-Hijdra, using a script?

Hmm... I forgot that your mod map is standalone. Well, then it's going to be problematic.
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However, I'll note that you don't necessarily have to focus on Ilm-Hijdra. There are other spell slots available, but it will still be a mod and it may conflict with other mods.