What Happens In Hellboy's Post-Credits Scene
The post-credits scene of Hellboy is a more intimate one than the mid-credits scene. Filmed as a continuous point-of-view shot, we once again see Baba Yaga, seated across the table from the camera. An infamous witch from Russian folklore, Baba Yaga offered Hellboy the knowledge he needed to stop Nimue in exchange for one of his eyes. It was a bargain Hellboy agreed to but then backed out of, claiming that he didn't need to give the eye to Baba Yaga at that moment since she didn't specify an exact time of delivery when they made their bargain.
An angered Baba Yaga explains all of this to an unseen character, through whose eyes the audience sees this scene. We hear this character respond to Baba Yaga's story with a deep male voice with a Russian accent. The scene ends with the witch promising the unseen man that if he can defeat Hellboy and bring her his one of his eyes that she will give him that which he wants most - death.
Koshchei the Deathless Explained
Though the character to whom Baba Yaga speaks is not identified by name or seen, he is easily identified by fans of the Hellboy comics as Koshchei the Deathless. Like Baba Yaga, Koshchei is a character from Russian folklore. And like the comic book version of Baba Yaga, Koshchei has a long and sordid history with Hellboy.
In the original Russian folk tales, Koshchei is described as a tall and bony man as well as a powerful sorcerer and shapeshifter. What makes him truly dangerous, however, is that he cannot die, having removed his own soul before hiding it inside a duck's egg. This egg was then placed inside a duck, which was placed inside a number of other animals and objects and locked away in a remote location at the ends of the Earth. This made Koshchei effectively immortal, as his body could eventually heal any injury so long as his soul was safe somewhere else, like Voldemort in the Harry Potter books.
In the reality of the Hellboy comics, Koshchei was originally an unusually skilled soldier who was betrayed by his fellow warriors and beaten near to death. Saved by a dragon and adopted as his son, Koshchei would face betrayal again after his great deeds won him the hand of a treacherous princess, who arranged his death rather than be married to a common soldier. The dragon resurrected Koshchei and sought to ensure his son would not die again by taking his soul and hiding it away in a duck's egg at the end of the world.
Unfortunately, Baba Yaga was able to learn where the egg holding Koshchei's soul had been hidden. With the egg in her possession, she was able to force him to do her bidding and turned him into the most powerful of her slaves. He would go on to face Hellboy in battle at her bidding, promised that she would finally allow him to die if he could successfully deliver one of Hellboy's eyes.
It seems likely that the Hellboy sequel will present this same battle, though Baba Yaga in the comics wanted Hellboy's eye because he had taken hers in an earlier conflict - not because he cheated her in a bargain as in the movie.