Mana
Term
Mana is the current amount of Spirit a creature has.
Spirit is the max amount of Mana a creature can have.
When a creature casts a Spell they spend Mana, which d RAWs on their Spirit. A 1st level Spell takes 1 Mana, a 2nd level Spell takes 2 Mana, and so forth. Once a creature's Spirit reaches zero, they cannot cast any Spells beyond Cantrips.