No one knows whence the first headless hunter came, nor how many there are. Several incidents have occurred over a few decades in Knuthe and the sleepy hollows north of the Inner Six, tales of shambling, headless bodies wandering the woods as if searching for something.
Furthermore, no one knows what to make of the cases following these incidents in which a member of a nearby village or community seems to grow taller or shorter, or to gain or lose mass, as if overnight. (And, it may be added, to gain a proclivity for high collars or fluffy ruffs.)
Headless Hunter (system-neutral):
Physical stats (including movement, HP, attacks) as a bipedal, sapient creature of your choice (often human, but an ogre would be a challenge, and a halfling would be hilarious!).
Mental stats nil (so no spellcasting), but any saving throws or resistances remain identical to the template creature.
Furthermore, it is immune to fear, charms, or other effects to alter its relentless drive to tear the head from another bipedal, sapient creature and attach it to its body. It may, however, be turned as an undead.
It has blindsense/tremorsense out to 20ft, beyond which it seems only to be able to feel out the general direction of a creature if it moves. Perhaps the deprivation of most if its senses has left its sense of touch extremely sensitive, and it is able to detect the slightest tremors in the earth or floor.
Headtaker: if the headless hunter reduces a target to 0hp, they immediately tear off (or cleave off, if a weapon is at hand) the target's head and place it upon their own suppurating stump, whereupon it binds with the body within a round (creating a ring of messy scar tissue). The original target finds itself in control of this new body, complete with its basic physical attributes and HP total (including any wounds), while it maintains its original mental stats, knowledge, and skills. The target's headless, bloodied body flees the scene, becoming a fresh headless hunter. Only miraculous resurrective magic can restore the target's head to its original body.
Relentless: if the headless hunter is reduced to 0hp, it does not perish; instead, one of its limbs is destroyed beyond recovery (roll 1d4 to determine which), and it loses a HD/level (but its HP count rises to half its maximum).
If an arm is destroyed, its attacks are usually impaired (exact mechanics vary based on the body template you're using, and the game system in general, but may include losing a multiattack, losing an attack bonus, losing a damage die, etc.).
If a leg is destroyed, the hunter may only locomote by crawling.
This effect may recur until no limbs remain to be lost, at which point the headless hunter is finally destroyed.
(If you're using a ruleset that awards XP based on slaying foes, it's recommended to award XP when "relentless" first triggers, and not force players to continue to brutalize this suffering creature any further just for the sake of the XP. The headless hunter is worth 1.25x as much XP as the base creature it was created from, at the GM's discretion; for example, a headless hunter created from a wizard or other spellcasting creature may be worth less, due to a significant loss in its original ability.)
Enjoy! If you use this, tell me how your players react!
(I had to try really hard not to just name this creature a "headhunter," haha.)