KOSCHEI
Russian Folklore
This is the name of a monster in the traditions and folklore of Russia. In some versions of the tales KOSCHEI, also known as KASHCHEI, is a DRAGON, but in other it is a male version of BABA YAGA. This monstrous being is described as immortal for the simple reason that KOSCHEI'S soul is kept hidden outside his body. In most tales the monster's undoing occurs when it abducts a beautiful young girl, in one version called Vasillissa Kirbit'evna, who finds finds out where the creatures soul is hidden. A hero, often BULAT THE BRAVE, is then given the task of retrieving the egg with the soul and confronting the creature. The final demise of the DRAGON is when the egg and the soul are smashed against KOSCHEI'S own head.
Source: Giants, Monsters, and Dragons: An Encyclopedia of Folklore, Legend, and Myth