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I know that, but what if the creature initially doesn't have a spellbook?
For example, I want to add the ability of invisibility, which can be activated, to ghosts, but they already have an ability to steal arrows, so that's why I had this question about adding a spellbook.
Or, if I add a second active ability, will the spellbook become available automatically?
Looks like I misled you a lot, lol. I somehow thought that it wasn't possible, although I haven't tried it myself before because I didn't need to.
It is possible, and it works.