A Voodoo doll is a mystical clay doll used to torture a certain person. If you placed it under water the person it resembles will eventually drown and if you bend the clay legs/arms backwards then you will be able to break this certain person's arms/legs.
Magic Golem

Magic Golem

In the film Edit

Lamia first used the Voodoo doll at the end of the film, as she fought the prince Septimus. First she brutally broke his arm and a leg. Then she throws the golem in a fountain. Septimus began to float in the air. He seemed to wanted to swim to the surface of the water, but as he can not control the golem, he does not reach it. At the end he drowns and joins his ghost brothers.

Secondly Lamia used the doll to fight Tristan. She fetched it out of the fountain and began to reduce the broken leg and arm of the seventh prince. Then she controlled the dead body of the seventh prince, so the body could fight Tristan. Tristan fought the dead body of the prince with his sword a while, but put an end to it by cutting a robe, which held one of the big chandeliers. The chandelier fell on the dead body of Septimus.