Kashaku (고문을안겨, torture; "one who inflicts torture") are dark creatures that are considered one of the foulest that inhabit the world. They feed off human happiness, and enjoy causing suffering and pain to mortals. They are also capable of consuming a person's soul, leaving their victims in a permanent coma-like state, and thus are often referred to as soul-sucking demons. They serve as the antithesis to the Seikōzoku; beings who wish to comsume the world into darkness and hatred as opposed to bringing change and peace.
They are generally described as "lifeless beings", their appearance consisting of what humans call "ghosts" or "monsters". Different than most beings of monsterious properties, there are four classes of Kashaku. The first and most common of all Kashaku are among the Ashisenshi Class (보병클래스, Footsoldier Class); they are depicted as hooded phantoms that don strange masks that conceal their presence. It is presumed that all Ashisenshi class Kashaku are all similar, if not exact, in appearance with eachother, besides the shape and design of their "skull" masks. The next above the Ashisenshi are the Yamashi Class (모험가클래스, Adventurer Class); while not too far off from the latter class in terms of appearance, this class is seen as more "monsterious" than "ghostly". However, at this class, they still don similar masks, but with much more unique craft to them then that of the Ashisenshi class. The third group are known as the Meishourai Class (다크미래의교실, Dark Future Class); they are by far the most different among the three main classes, for they possess a more humanoid like appearance to them. Like the latter two however, they still don masks on their head with the added bonus of strange horns and/or antlers on them. They possess the greatest power among the first three classes, and are very rare to ever spot or find.
The fourth class are quite strange, and have yet to be seen. All that is known is that they are not refered to as Kashaku; being named Fuketsu (더러움을, to taint, to make impure). While still unknown, it was said that Hwanin Unagi lost his life to a Fuketsu in battle, though this is still unproven.