Guardians are powerful, invisible creatures that have historically been summoned and bound by skilful priests. They do not communicate, but follow directions well, making them an obvious choice for a magical guardian.
Little is known about the physical features of the guardian, but they are vicious melee fighters who have a poisonous bite. They can also breathe fire.
Guardians are not demons, despite the methods used to summon them, and they are not undead, despite their many commonalities with shades. They are, however, sometimes employed in conjunction with black shades, since both creatures are invisible.