Maximum magic
The Grimoire unlocks when Wizard Towers reach level 1. Your magic cap depends on how many towers you own and their level: max magic = floor(4 + T^0.6 + 15 × ln(1 + (T + 10 × (L − 1)) ÷ 15)), where T is the number of wizard towers and L is their level.
The formula grows slowly, so doubling your towers adds only a little magic. Leveling towers with sugar lumps helps too. The planner shows your cap and how long a full refill takes.
Spell costs
Each spell costs a fixed amount plus a percentage of your maximum magic. Force the Hand of Fate, for example, costs 10 magic plus 60% of your cap. Because the percentage part scales with your cap, raising max magic does not make spells cheaper relative to it. It mainly lets you cast more than one spell before refilling.
The spell list shows each spell's cost at your magic cap, whether you can cast it with your current magic, and roughly how long until you can.
Recharge and double casting
Magic refills faster the fuller the meter is, so letting it drain to zero is the slowest way to recharge. A popular strategy is to keep your magic cap just high enough for two Force the Hand of Fate casts in a row, usually by selling wizard towers right before the second cast to lower the cost. The planner's notes cover the tower counts that make this work.
Backfires
Every spell has a chance to backfire, which turns its effect into something bad (Force the Hand of Fate spawns a wrath cookie instead). The base backfire chance is 15%. It rises with golden cookies on screen and some Pantheon choices, and Diminish Ineptitude lowers it for a while. The outcome table shows what each result looks like. For exact upcoming outcomes, use the FtHoF Predictor.
Frequently asked questions
What is the best spell in the Grimoire?
Force the Hand of Fate, because it spawns a golden cookie that can start or extend a combo.
How do I unlock the Grimoire?
Level your Wizard Towers to level 1 with a sugar lump.