Human skeletons have long been used as a source of witchcraft in the East and West. In the East, I suspected the use of human skeletons especially the skull was a misconception of left-hand tantrism.
It is interesting to point out that tantric practitioners only view human skeletons as tools for music or ritual instruments and food containers. The uses of skeletons have never exceed this context in both Hindu and Buddhist tantrism.
When tantric practices were somehow introduced to the East, mainly China; the conservative Chinese treated human remains as a taboo and evil. So it was perhaps under this circumstances and cultural differences that the Chinese and later Indochinese started to make use of human remains in black sorcery.
The above mentioned 'skeleton ghost' must have something to do with the Tibetan dharmapala (protector spirits) named 'kinkara'. Kinkara is normally depicted as a couple of male and female skeletons dancing together. To Tibetans, this dharmapala is good as it protects the teachings of Buddha. However, to the outside world; Kinkara is evil... How can skeletons dance unless there is evil forces at work?
Come back to the topic of skeleton ghost... This is a black art said to be originated from the western part of China and then proliferated to Indochina and Taiwan. There are many forms of this ritual some only use human skulls while others use the whole skeleton.
Old folks believe that human souls would remain in their remains long after they died. So by holding these skeletons as ransom, the human spirits will do whatever sorcerers ask them to do. In old days, skeleton ghosts are normally used as informants and information provider. Some blind fortune tellers were believed to be masters of these skeleton ghosts.
The methods to 'make' a skeleton ghosts varies according to places. A more general method is first to find the corpse of a kid below 12 years old. This is a very difficult task nowadays but in old China and Indochina, corpses were plentiful especially during time of unrest.
Now this dead kid must be 'treated' and the methods varies too...
In Vietnam, the body of selected kid is buried. His grave will be watered with rice wine once a day for 100 days. Then the remains of the kid is exhumed and washed with rice wine again to remove remainder of flesh and tissue.
This completely cleaned skeleton is wrapped with a piece of black cloth and then worshiped on an altar. Incense must be burnt twice a day with some food offerings. When the sorcerer wanted to know something, he will take a cane and strike the skeletons repeatedly until the spirit 'talks'.
In old mainland China however, the remains of the kid is watered with vinegar everyday until all of the flesh rod leaving the skeletons. This skeleton is collected into a silk bag and worshiped in a dark corner. When the sorcerer needs some information, the ghost of the kid will be summoned with mantra and burning of 'fu' (talisman).
As far as I know, some Thai archan (masters) also secretly dig out remains of children for black magic purposes too except that these sorcerers prefer to wait until the flesh has decayed away instead of removing the flesh with own hands.
Nowadays, the use of human skeletons is replaced with willow wood effigy (柳灵童) and the responses are weak compared to actual remains as my fellow sorcerers told me. There were still some willow wood effigies practitioners in the 80's. I have not come across any one still holding to this sort of practice during the past decade.
