Nukekubi, meaning Detachable Neck, also written in Japanese Hiragana characters as "ぬけくび" or Japanese Katakana characters as "ヌケクビ" (But still read as "Nukekubi") are monsters found in Japanese folklore. By day, nukekubi appear to be normal human beings. By night, however, their heads detach at the neck 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.
While the head is detached, the body of a nukekubi becomes inanimate. In some legends, this serves as one of the creature's few weaknesses; 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 Mike Mignola's Hellboy, nukekubi were featured in the all time popular short story, "Heads".