Stress Level

Stress Level, sometimes called Stress Limit, and abbreviated as SL, is a measure of how much magical energy a mage can control within a period of time before beginning to suffer physical consequences. It is utilized by practitioners of arcane, nature, psychic, and occult magic, as well as a few users of eldritch magic.
SL by default starts at 0. Whenever a mage casts a spell, their SL rises by an amount equal to the spell's level; a level 3 spell increases SL by 3, for example. This assumes that the ambient Mana matches the color required for a given spell. If a spell requires 2 red mana but only blue and yellow mana are available, the caster must spend 1 more SL per unit of mana to be converted; this also applies to FP mages, though they only need to raise their SL, not expend extra FP, to convert mana. Indigo mana is considered "raw" mana, and so can be used for any color, but since it doesn't exist in the material plane, this is only possible while in the nether realm. If a mage has the Ki to Mana talent, they instead use CP to convert colors, instead of raising their SL.
While not in combat, SL reduces every round (10 seconds), the percentage amount being determined by the character's Resilience, which by default is equal to [1 + (((Intellect + Wisdom + Personality) ÷ 30) + (Proficiency bonus ÷ 2)) ÷ 2]%. If someone has the Lesser Battle Resilience talent, then they retain their Resilience in combat unless being Threatened (they're within an enemy's melee striking distance and vulnerable to Attacks of Opportunity), under any Stupefying debuff, or if their SL exceeds 100%. With the Greater Battle Resilience talent, only exceeding 100% SL cancels Resilience during battle.
By default, someone's Stress Limit is equal to [(Intellect ÷ 5) + (Wisdom ÷ 5) + (Personality ÷ 5) + (Level ÷ 2) + Proficiency bonus]; all classes have a Stress Limit, though only the above-mentioned mages utilize it. One can potentially go up to 200% capacity before dying, but the higher above 100% one's SL rises, the more stressed the mage becomes, the more issues arise, and the more damage the mage suffers for using magic.

Stress Level Ranges

0% - 100%

No ill effects. This is the range of stress the mage can handle without any adverse effects under normal circumstances.

100% - 125%

Minor Stress. While in this range, every time a spell is cast, the caster must make a Concentration check of [10 + (1 + spell level × 3) + (number of different components × 2)], or else lose the spell; if lost, SL still rises. Enemies attempting to make Attacks of Opportunity against the caster while Casting Defensively get a +10 Circumstance bonus on their Awareness checks to recognize a spell is being cast. Psychic casters can no longer nullify psychic backlash for their spells even if they meet the Personality requirements. The caster cannot Focus to maintain spell effects. Finally, if the caster loses a spell, they must make a Spirit saving throw of [25 + (1 + spell level × 4)], or else suffer Mystic/Nether damage equal to [3 + (1 + spell's level × 2)].

125% - 150%

Moderate Stress. While in this range, the Concentration check casters must make becomes [20 + (1 + spell level × 5) + (number of different components × 3)], they must always make a Spirit saving throw to avoid damage, and the enemy AoO Circumstance bonus becomes +15. Psychic backlash always occurs after casting a spell in this state.

150% - 199%

Major Stress. While in this range, the Concentration check casters must make becomes [30 + (1 + spell level × 8) + (number of different components × 4)], and they always suffer damage for casting. Enemies now get a +20 Circumstance bonus on their AoO checks. Finally, the caster must make a Constitution saving throw of [25 + (1 + spell level × 4)] after attempting a spell, or else die outright; they get no Death saving throw.

≥ 200%

Death. If at any point SL reaches or exceeds 200% capacity, the caster dies, with no Death saving throw.

Type

Metaphysical, Supernatural