The Kagebi (陰火, lit. Shadow Fire) are bluish, flame-like mushi which feed on the body heat of living organisms. Their true form lies beneath the flames and is called a Hidane.
Their life cycle has two known stages: the larval stage, which resembles a fast-growing blue weed with squirming roots characteristic of mushi limbs; and the adult form, called a "ghost fire".
Kagebi in their larval form
The larval form relies on the natural tendency of humans to burn unfamiliar things, particularly if threatened; to achieve this the larval form breeds and spreads rapidly and can release a toxin to kill surrounding flora. When exposed to the heat from the fire, they release their adult forms, which then seek out narrow spaces such as pots to nest in. At this time, they may either wait in their chosen hiding place for unwary humans to come close, or they may take up residence in an already-established fire. When animals and humans draw near for warmth from what they perceive to be an ordinary fire, the Kagebi suck body heat away from them causing them to freeze to death. Additionally, any food cooked by the false heat of the Kagebi causes internal frostbite when consumed.
Weakness
The flames of mature Kagebi can be used to safely burn the larval weed form. If the Kagebi has managed to take up residence in a living host, water boiled by the flames of an adult Kagebi can be consumed to burn the larval form out over time. The mushi that live inside the "ghost fire" of the mature form can be crushed and destroyed.