When you gain this companion, choose a 'spell' domain card availble to you that you have not yet added to your vault. This domain card is stored by the Grimoire Pal. Once per short rest, you can swap any spell card in your loadout with the card held by the Grimoire Pal. When you swap cards, make a Spellcasting reaction roll with a Difficulty of 15. On any success, the Grimoire Pal casts the spell that it was storing at the nearest target. If the spell card has multiple options (eg. Codex Domain) it is up to the GM which effect is triggered.