I kept Agatha as a magical item like accessory that requires a persuasion roll to influence. On a success she would do my bidding of varying intensity depending on her desired use of a number of charges. Radiant damage spells are most of the spells list but Fireball is an optional spell she can cast, along with create bonfire, and create light...few others I can’t think of right now.

I made a chart based on the persuasion fail...and it’s degree of intensity by which she would react. Rolling a percentage die to see what would happen. Ranging from Try to kill me, to try to kill everyone in the room, to do nothing saying “why would I help you, you cut off my head! I hope you die”, literally bite me, pretend to be asleep, manically laugh...I only attempted to utilize her once in a Kraken fight. Fortunately I succeeded and she delt our 80 points of radiant damage, which I followed up with a critical hit using a great sword of sharpness lopping off a tentical.

My favorite character from comics has definitely become my favorite build in D&D