Enemy's Head
Many cultures place great emphasis on their warriors bringing back the heads of their enemies. The Celts, the Norse, and many other traditional tribal groups, are all tied closely with this. They believe the spirit of the person resides in the head.
Manufacturing process
In the instance or retaining the skulls, the flesh is boiled away to allow access to the skull. In keeping the whole head it can be placed in oil to seal it. In the instances where the skull is removed the tissue is removed from the head with a knife and oils rubbed in to the tissue to stabilize it.
History
The Celts, the Norse, some hunter-gatherer tribes, and other societies are the usual societies to head hunt. Some go back for centuries if not longer and divergent reasons. One reason though stands out over time, the head hunters do it to terrify their opponents in battle, and to show their expertise in killing. Many of the same groups also use it as they believe it will bring them power in battle by taking the heads of other killers, and noticed by the gods by the amount of enemies they have slain.
In many cases this has became less & less as tribes have settled down, with standing armies in many regions, and clear laws & religious beliefs that have changed the moral & ethical attitudes of the masses.
Significance
For those that still practise this, it is highly prized, with either the skulls of the killed being kept in a personal space, or the heads being placed in a shrine to the gods. A few keep the heads with the skulls removed on themselves to show their prowess. People like the Celts keep a few prized skulls in a specific chest or shrine, while the Norse who practise it, will keep them at a shrine to a specific god.
"Disgusting practise that, head hunting. Seen it done in the far north west around Brittany. Many of the old ways of the Celts, especially the Bretons & Gauls have close ties to the Old Ways there. They keep the skulls of slain enemies removing the soft tissue, placing them around their abodes. Especially powerful individual that are killed take pride of place being placed to one side away from the others, or in a small shrine, or even a chest. These latter skulls are took out and shown to visitors who the Celts deem worthy to show of their prowess in battle. Make sure if you go that way, don't fall to the blade of one of them heathens."
Father Tossil to an adventuring party looking to carry out a job for him in Brittany.
Item type
Trophy
Rarity
Very Common in some societies, ranging to non-existent in others.
What a very dark and cool culture and religious traditions it was a very enjoyable article
Worse thing is it's based on actual traditions. With a bit of BS thrown in for good measure