Viktor Vasnetsov, "Kashchey the Immortal"
Andreas Johns points out that it's ironic that Koschei is sometimes referred to as "Koschei the Deathless," because in the stories in which he appears, he is inevitably killed by the hero. Yet the term "Deathless" could refer not just to the fact that he is difficult to kill, but the fact that he keeps his "death" outside of himself. It's interesting that the egg should have such negative connotations, since usually eggs are associated with life/birth/resurrection, but the egg can also be said to contain his soul. In one tale, the hero Vasilii uses the same egg to kill Koschei as to revive his birth father, so sometimes (although rarely) the egg can have life-giving properties.
The storing of the soul within multiple objects gives Koschei protection, as it's more difficult to find and to get to, but the reciting of the egg's location has a lilting rhythm to it that is more obvious in Russian but even evident in English ("in an egg, in a duck, in a hare, in a chest..."). In fact, the Russian phrase that often begins the series, "On the sea, on the ocean, on Buian Island" also occurs in East Slavic incantations. Johns suggests that tale tellers were either familiar with incantations, or practitioners of spells themselves. Irina Razumova suggests that both genres come from a culture which believes in the magic power of words.
Although the Death Egg is almost always Koschei's, there are exceptions; in one tale type a maiden's love is contained in the egg hidden within the series of objects, and in one tale it's Baba Yaga whose soul is hidden in the egg. But whoever is hiding their soul in eggs, the hero of the tale is bound to discover the information and smash the egg anyway. We can look at Koschei as a reminder of the inevitability of death-you can try to prolong life, but it will catch up to you in the end. From the other perspective, it's encouraging to see that even the most difficult obstacles can be overcome by the hero, and good triumphs.
Baba Yaga: The Ambiguous Mother and Witch of the Russian Folktale
Faberge Eggs-a royal Russian Easter tradition