You get your grimoire spells by having them appear as you get stronger. The grimoire chooses its' user by itself, and has different elements that come from the 4 basic elements: earth, fire, water, and air. Branching elements such as darkness or light exist as extensions (for the purpose of simplicity there will only be the biggest elements from Black Clover. Feel free to make more of your own.).

Pick an elemental affinity from: Air, Water, Earth, Fire, Light, Dark.

Spell Write

Starting at 2nd level, your grimoire gains one 2nd level spell, or lower, from either-

Air: Gust of Wind, Knock, Levitate

Water: Create or Destroy Water, Misty Step

Earth: Spike Growth, Mage Armor, Entangle

Fire: Burning Hands, Flame Blade, Heat Metal

Light: Faerie Fire, Guiding Bolt

Dark: Bane, Blindness/Deafness, Darkness

This uses normal wizard spell slots. At 5th level you get a second one.

Spell Tweak

Whenever you cast a spell that has to do with your element, as judged by the DM, you can increase the range of them by 10 feet. The spell can also avoid allied creatures if you choose to do so with the spell.

Extra Cantrip

At 6th level, gain an extra cantrip depending on your affinity.

Air: Gust

Water: Ray of Frost

Earth: Druidcraft

Fire: Firebolt

Dark: Eldritch Blast

Light: Light

Utility Ability

At 10th level, you can use a utility spell of level 4 or lower. You can use this once a day after a long rest.

The spell has to have no ties to any element, and when you cast it, it has a flare from your element in it and changes depending on your own personality.

Last Effort Spell

 

At 14th level, you can gain the benefits from Arcane Recovery without a rest. This will still use up the use of Arcane Recovery.