Kakuja

A Kakuja (赫者, red one, kakuja) is a kind of ghoul with a transformed kagune that clads the ghoul's body. Though it is rare, it can occur through repeated cannibalization. When a ward has a higher ghoul to human ratio, cannibalization is more common due to turf wars and limited food supply.

Sometimes, the transformation is incomplete, and such individuals are called half-kakujas (半赫者, han kakuja).

Biology
It is said that through eating, a ghoul absorbs rc cells, and this increases the concentration of Rc cells within a ghoul's body. If the concentration becomes high enough, it may trigger a mutation and lead to the formation of a new kagune.

A kakuja can further evolve by continuing to cannibalize their kind. For instance, while the One Eyed Owl had a human-shape and size before its disappearance, it eventually developed into a much larger and monstrous form during the Raid in the 20th ward.

The transformation seems to be affected by psychological factors as Ken Kaneki's kakuja takes the form of a centipede, resembling the Red-headed Chinese Centipede Yamori used to torture Kaneki by putting it in his ear. Also, earlier Kaneki compared himself to a protagonist of the novel The Metamorphist who turned into an insect-like creature while picturing a centipede in his mind. Ken Kaneki's kakuja mask also grew to resemble the One-Eyed Owl's mask, with the former holding the desire to keep attention towards himself and away from the latter for personal reasons.

It should be noted that members of the Tsukiyama Family have a birth defect where cannibalism has little to no chance of triggering this mutation in their Rc cells due to consanguineous marriages, thereby preventing kakuja mutations.

Powers
Apart from the predatory strength ghouls commonly possess, a kakuja has a special kagune that clads the ghoul's body and functions like an armor. The kagune amplifies the kakuja's strength or diversifies their abilities, which makes fighting one of them especially hard.

However, using a kakuja kagune is nearly impossible because most users tend to lose their minds while fighting with it. It is unknown why some ghouls are able to remain sane and others are not. One reason may be that all unstable/insane ghouls using kakuja are half/incomplete as full kakuja are not observed to have any problems maintaining control.

It appears that a kakuja kagune can form a mask over the user's face if need be.