Here is the core of the magic system. Spell crystals alone function sort of like scrolls (consumed when used, no mana cost), or they can be inserted into foci for repeated use (such use requires mana). I think this creates an interesting and diegetic magic economy while not compromising either simplicity or flexibility. They system for sustaining spells also reigns in the complexity of stacked effects without artificially limiting the power of spells. Right now, I see black and white mages starting with a wand (focus) and 3 crystals of 1 mana spells and red mages starting with a wand and 2 crystals of 1 mana spells.

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Magic

Magic is the ability to use the power of mana to reshape reality. On its own, however, mana is raw potentiality. It is dangerous and overwhelming. Only some character classes, notably mages, have the ability to use magic.

Types of Magic

Magic comes in two varieties, black and white. Black magic is mostly destructive and offensive, while white magic is mostly supportive and defensive. The two types of magic are different enough that they require different skills to master. Black mages can only cast black magic and white mages can only cast white magic. Red mages learn how to manipulate both kinds of magic, but this generality comes at the cost of specialization.

Spell Crystals

In order to focus and tame the dangerous power of magic, mages have learned how to encode spells in special, alchemically prepared crystals.

Spell crystals may be used directly to cast the spell contained. No external mana is required, as the spell draws upon the mana originally used to encode the spell, but using a spell crystal in this way consumes it. In other words, when using spell crystals directly, spells may be cast “for free,” but this uses up the crystal, leaving only a worthless, burned out husk.

Spell crystals are considered insignificant items for purposes of encumbrance.

Creating Spell Crystals

Mages can manufacture copies of any spell crystal they have access to, though the process requires expensive material components (50 GP per point of mana spell cost) and takes a full town turn. Sometimes special components, such as unrefined meteor crystals, may be used in place of purchased alchemical reagents for spell crystal creation.

Foci

Spell crystals, on their own, are consumed when used. However, with the help of a focus, mages can use spell crystals multiple times. Foci allow a mage to supply the mana required for spell casting themselves rather than drawing on the inherent mana infused in the spell pattern. Most foci are wands or staves, as something about that shape helps facilitate the channeling of mana. Each focus may hold one spell crystal. Traditionally, all apprentices of the three primary mage orders are given a wand (that is, first level mages begin with one wand focus). Each focus carried is encumbering. Special foci exist that can add benefits to the casting of certain spells. For example, a particular magical staff might add extra damage to fire spells that are cast using it as a focus.

Modifying Foci

Attaching a spell crystal to or removing a spell crystal from a focus is a complicated and delicate procedure, and may only be done during town turns. Any number of foci may be modified (within reason), however, and this does not consume an entire town turn.

Mana

Mages often need to draw on their own personal mana to cast spells, such as when using a focus.

Recovering Mana

A character’s mana is replenished following a night of restful sleep. Certain items (such as mana potions) or spells (such as the black magic spell leech) may allow limited mana recovery between rests.

Temporary Mana

Some items or effects may provide temporary mana. This mana functions like normal mana, but should be tracked separately, and evaporates after combat or one exploration turn.

Casting Spells

To cast a spell, a character must have the ability to use the type of magic in question (black or white) and either consume a spell crystal or spend mana to cast a spell through a focus. No magic is possible without spell crystals.

Magic Defense

PCs use wisdom saving throws to determine their magic defense, but most NPCs have a static magic defense (10 by default).

Spell Checks

Offensive spells require a spell check to determine their effectiveness. Mechanically, this is an intelligence check opposed against the magic defense of any targets.

Spell check: 1d20 +level +INT vs. magic defense

This functions sort of like an attack roll, but for mages, though there are a few important differences, the biggest being that spells that “miss” can sometimes still affect the target, though in a lesser manner. For example, spells with the save-half property still inflict half damage on a miss. For spells with multiple targets (such as area effect spells that affect an entire melee), roll once and compare that roll to each target’s magic defense score to determine the outcome.

Add INT to damage done by spells (to the whole damage, not to each die). For example, a black magician with INT +2 does 2d6+2 damage with a blaze spell.

Sustained Spells

Some spells have effects that persist. Only one persistent effect may be maintained by a mage at any given time; sustaining a spell does not consume any additional mana beyond the initial cost. If another spell with the sustain property is cast, the previous sustained effect ends. Instantaneous spells (that is, any spell that does not have the sustain property) may be cast while sustaining a spell. For example, a black mage that is sustaining the fly spell may cast a shock spell from the air, but if they cast a darkness or invisibility spell (both of which also require sustaining), then the fly spell will end. Sustained spells also end if the caster becomes unconscious.