[quote=@BrokenPromise]
[@TheWendil] Okay, so I have some questions. 

I pretty much have everything but the actual magical girl part figured out, hence where the questions come in.

So I was thinking about doing was an alter tale based on the Fenian Cycle. Yes, Cuhullin is a total chad, but I really like the stories about the more "human" Fianna clan members led by Fionn mac Cumhaill, or Finn McCool as he is commonly called in English.

My issue is that there is no single story that these dudes appear in. They're in several poems written in a body of literature called the Fenian Cycle, which is a collection of early irish writing that spans from the 9th to 14th century. So my question is for a grimoire would it be acceptable to use a collection of stories like the Fenian Cycle that more or less follows the exploits of the Fianna, or would I have to pick a single poem/story to draw inspiration from?

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The later option is fine. That's pretty much what many grimoires based on old mythology should be anyway, since those stories always tend to be composite to some extent.

In a way, it even opens the possibility of more than one grimoire drawing from the same rough source, like Camelot and Merlin both originating from the Matter of Britain. Most mythological cycles can be used in this way, and actually make more sense if so, so the idea of a character with an anthology like grimoire is fine in these cases.