If you're like me, you first saw these freaky creatures of Japanese folklore in the pages of Mike Mignola's Hellboy - they're featured in the short story "Heads", and also in the animated piece, Hellboy: Storm of Swords (pictured). Mignola loves classic folklore and the pages of Hellboy are full of unusual creatures.
Like many supernatural creatures in Japanese folklore, it's ambiguous whether Nukekubi are demons, cursed humans, a kind of undead, or spirits. They appear almost entirely like normal humans during the day, and often live in a family group of 2-5 members, somewhere remote. When night falls, their heads separate cleanly from their bodies and fly off in search of human prey. In human form, a Nukekubi can be identified by a thin red line along the seam where the head cleanly separates from the body each night.
In the Hellboy story, the Nukekubi offer Hellboy hospitality at their remote villa; when he wakes in the night, he sees all the headless bodies sitting in the parlor, grumbles in the usual fashion, and then the heads start hunting him.
AC as leather, HD 4, attack 1 bite, damage 1-6, MV 12, ML 10, AL Chaotic.
Nukekubi emit a frightening scream when they fly into combat, causing Fear (as the spell), to panic their victims and break a group apart. They'll track a target into the night, biting, rending, and tearing the flesh.
A Nukekubi head takes damage from normal weapons, but isn't destroyed when it reaches zero hit points; instead, the head is forced to flee and return to it's body. The only way to destroy a Nukekubi is to prevent the head from rejoining the body by dawn, by hiding the body or locking it someplace secure.
Nukekubi are another cool monster for the future Spirit Island campaign, and also a contribution to Tim's weekly Monstrous Monday in honor of Halloween: