Sorcerers have a secondary ability score, Magic Points, which represents a reserve of magical energy they can use to power their spells.
Mystics have no such predictable measure of how many spells they can cast a day. Instead, they must check whether they do, or do not, suffer Psychic Fatigue after casting each spell.
Sorcerers cast spells by expending Magic Points equal to the level of the spell they wish to cast. If casting the spell would reduce the sorcerer's current Magic Points below zero, the spell will fail.
Magic Points are automatically restored at midnight.
After casting a spell, a Mystic does not deduct Magic Points, but instead must test for Psychic Fatigue by rolling a twenty-sided die (d20). If the result of this roll is over the sum of 13, plus the mystic's Rank, minus the spell's level, then the mystic is fatigued and may cast no more spells that day.
1d20 > 13 + Rank - Spell Level = FATIGUED!
Mystics recover from Psychic Fatigue at dawn each day.
Lyona, a 3rd rank sorcereress with a full complement of 12 Magic Points, casts Warding, a second-level spell, at a cost of 2 Magic Points. Lyona now has 10 remaining Magic Points.
Caedmon, a 2nd rank mystic casts Invigorate, a first-level spell. Caedmon needs to roll 14 or less on a d20 to avoid psychic fatigue. Either way, the Invigorate spell takes effect.