Wikipedia:
"Polong is Malay for a spirit enslaved by a man (most of the time) for personal use.

Polong is said to have been created from the blood of a murdered person and this blood is put into a bottle for one to two weeks before the spirit is invoked with incantations and magic spells.

After two weeks, the owner will start to hear sounds coming out of the bottle. It is the sound of crying. By then he should cut his finger and drain the blood into the bottle to feed the demon. This is the sign of allegiance and of loyalty to serve the master. The blood which feeds the demon is said to have tied both parties together: one as Master and the other as the servant.

No one has ever illustrated the figure of the demon but all agree that it is evil and hideous."