Title: Kashchei the Deathless
Type/Species: Great Serpent
Origin: Russian Mythology and Slavic Folklore
About Kashchei / Koshchei / Koschey / Katschei / Koshchey / Kashey
In some tales, Koshchei is a wizard that is much like the hag Baba Yaga, but in many Slavic tales Kashchei is a dragon of tremendous power.  A few stories also refer to Koshchei as an evil, immortal snake. 
There are many variations of Koschei's story, namely the names and relationship between the two other characters involved. Some stories name the hero, others simply title him as "the prince." Sometimes Koshchei captures the hero's mother and, more often, the hero's wife or bride-to-be.  Often Koshchei took the form of a whirlwind or a heavy fog to capture the young woman. 
Kashchei's full title was "Kashchei the Deathless," for he was considered to be invulnerable.  His immortality was no accident. Kaschei kept his soul hidden outside of his body, so no harm done to him could kill him. However, should anyone harm whatever he stored his soul in, he would die. 
There was a particularly wily maiden that Koshchei kidnapped. She used false and flattering words to convince him that she cared for him, even loved him. She asked him how he could die, and where he kept "his death." In the story, the term "his death" is written instead of "his soul" or "life force," which appears in some later translations.  The first two times the warlock dragon answered her question, he lied. She pestered and wheedled him more, telling him that if he really loved her, he would tell her truly where he hid his death. 
Depending on the story, Koshchei explains the true resting place of his soul. In the versions where he is a dragon, he tells her that he has hidden it in "a small stone found in the yolk of an egg, which is inside a duck, which is inside a hare, which is inside a stone, which is on an island."  The young maiden informed the hero, usually a prince, who finds the rock and smashes it against Koshchei's head. 
Bulat the Brave is the hero most commonly attributed with this dragon's death.
Kashchei is a boney, snakelike dragon that stands on two legs and has two arms.