Fae are defined as creatures that possesses auras. Most fae radiate a force called either and when enough are in close proximity this effect causes ambient "mana pool" to exist.
Fae they do not age beyond maturity, are immune from mundane deceases, and are resistant to environmental extremes may be harmed or killed by violence, accident or starvation while in a adequately powerful mana pool. The requirement increases with the chronological age of the fae.
Fae that spend long enough outside an adequate mana pool can begin to suffer mana deprivation and resume aging process at the rate they matured at until reentering a adequate mana pool or death due to old age occurs.
Fae are extremely adaptable, undergoing changes to make them more suited to their environment within two generations. A slower change is the subtle trend for fae to take on elemental aura affinities based on their surroundings. Cases of both are prevalent with surface fae that moved to the underworld.
Most fae have the ability to use mana arts, which are super-natural abilities that further differentiate them from non-fae. Most are based on their aura affinities, though sapient fae have developed abilities outside this limitation called High Arts.
Fae are vulnerable to an extra-dentitionial force called nether and may have their aura damaged by it. A fae's aura may be "bitten" and consumed by nether beings known as a demon. Demons may also consume a fae's aura completely and possess fae body, usually mutating to become more like its original form. Demons may also "taint" a fae, where a weak juvenile form of the demon called a seed is permanently joined into the host fae's aura.