Secret Arts
The basis of using magical abilities is the art of concentration. Individual methods may differ in the way of concentration, uttering certain words or only mental practices, but the basis is always total concentration and mental exertion.
Mana
The basic unit of the cost of spells and mental skills is the mana point (MP). Each spell costs one or more mana points. If you exhaust all your mana, you can no longer cast.
Mana
Wil + 5 + number of feats in magical skills
Rest
If you sleep well (8h), sleep restores 1 MP for every full 10 MP of your maximum.
The rate of regeneration can be further improved by feats or by resting at magical places.
Using mana crystals
For your spells you can use pure mana in crystals. It is, however, very rare and almost never found in the world. Only very experienced alchemists can make mana crystals artificially. If you find a mana crystal, you will recognize it at first sight. When casting a spell or effect, it is enough to hold the mana crystals in your hand; each one gives you 10 MP to pay the cost of the spell. Unused mana dissipates and may cause a side effect.
With a mana crystal you can also replenish your own mana, but not more than your maximum. Replenishing takes 1 minute of concentration and thus cannot be done in combat.