Nukekubi (抜首) are yokai found in Japanese folklore.
By day, nukekubi appear to be normal human beings but at night, the head and neck can detach from the body and float in the air while the latter remains inanimate. The only way to tell a nukekubi from a normal human being is a line of red symbols around the base of the neck where the head detaches. However, this small detail can be easily concealed beneath clothing or jewelry.
By day, nukekubi often try to blend into human society. They sometimes live in groups, impersonating normal human families. By night, however, their heads and necks detach smoothly from their bodies and fly about independently in search of human prey. These heads attack by screaming, (to increase their victims' fright) then closing in and biting.
If a nukekubi's head cannot locate and reattach to its body by sunrise, the creature dies. Legends often tell of would-be victims foiling the creatures by destroying or hiding their bodies while the heads are elsewhere.
In recording on particular folktale of the nukenubi for his book, Kwaidan, Lafcadio Hearn misidentified them as Rokuro-Kubi, an error that also appears in the Fighting Fantasy book, Sword of the Samurai. Rokuro-kubi are another type of Japanese monster entirely; instead of heads that completely sever, the rokuro-kubi have necks that stretch to enormous lengths during night-time.