Magical auras are perceived as mist, generated from within any magic object or magically gifted person. The colour of the mist depends on the school of magic that generates it, and the thickness depends on the strength of the contained magic. The intensity of the mist is referred to as the Magic Class of the object or entity.
Most characters have a base MC of 0, meaning that they are entirely free of magic.
If a character is capable of learning magic, it’s base MC is increased by 1. If it is a member of a non-human race, it’s base MC is increased by 1/2. Monsters, and playable monster races may have a higher base MC. Characters with permanent magical traits, whether good or ill, have their base MC increased based on the approximate spell-level of the trait (see below).
When a character casts a spell, is effected by the magic of a spell (for example, being augmented by Cat’s Grace, rather than taking damage from Fireball), invokes a magic item that casts a spell, or wears a magic item that has a continuous spell active (including any enhancement bonuses beyond masterwork quality), these spells increase the character’s MC based on the spells’ levels.
Spell level/MC Increase
Merely having memorized a spell or carrying a magical item without activating it increases a character’s MC. Passive spells increase MC by 1/10 of the value of an active spell of the same level. Memorized spells, or unused spell-slots for spontaneous casters, are considered passive spells, as are invokable magic items or magic items carried but not active (usually, wearing or invoking a magic item activates it, and taking it off or dismissing the invocation deactivates it).
These increases are cumulative (thus being enchanted by a 5th level spell and casting a 3rd level spell increases a character’s MC by a total of 11 points). When a character’s MC decreases, it does so at a rate of 1 point/minute.
Auras can be detected by creatures with the Detect Magic ability. This ability can be innate, as in some monsters; granted by class, as the Inquisitor, or achieved by magic, as with the Detect Magic spell or Goggles of Magic Sight. Unless otherwise noted, detection of auras works as follows:
For each aura in range, calculate that aura’s MC. Reduce each MC by 1/2 point/5 feet the aura’s origin is away from the detector.