When a caster prepares spells, they gain a stack of Arcane Fatigue. Most casters can prepare spells only ONCE per long rest by default. This limit to prepare spells is known as the Arcane Threshold. Wizards for instance, study spells through academic means, often using their spellbook, and naturally have a higher Arcane Threshold, and can prepare spells more times per long rest, or switch their switch their prepared spell list.

This limit can be increased through specific feats or class features. Once a caster’s arcane fatigue reaches their maximum allowable preparations, their essence can no longer attune to new magic until a long rest.

1. This restriction applies only to casters in a campaign, in a campaign a character can prepare spells as many times as needed.
2. All Arcane Fatigue stacks expire after a long rest.
3. The maximum amount of Arcane Fatigue stacks any non-godlike creature can take is 10.

Preparing spells requires 10 minutes of uninterrupted focus, during which the caster must be out of combat. While preparing, the caster becomes immobile, honing their senses and attuning their essence to the flow of magic.

1. For Wizards, this might be studying their spellbook.
2. For Priests this might be consoling their higher power(s).
3. For Warlocks they might consult their patron.
4. For Sorcerers they are realigning their body and essence.