Which immortal sorcerer in Slavic folklore hides his soul inside a needle?

The immortal sorcerer whose soul is hidden inside a needle is Koschei the Deathless.

Koschei—also spelled Koshchei or Kashchei—is an archetypal male villain in Russian and East Slavic folklore. His immortality depends on hiding his “death” outside his body, most famously in a needle. In the best-known version, the needle is inside an egg, the egg inside a duck, the duck inside a hare, and the hare inside a chest buried or concealed on the distant island of Buyan.

The layered hiding place turns the villain’s supernatural protection into a quest: the hero must discover the chain of objects and break it in the correct order. Koschei often appears as a rival who abducts or imprisons a heroine, while a male hero attempts a rescue. The tale “The Death of Koschei the Deathless” was collected by Alexander Afanasyev and later popularized in English through Andrew Lang’s The Red Fairy Book.

A common mix-up is assuming every version uses exactly the same objects. Folklore varies, but the needle-and-nested-containers motif is the defining feature. Ivan Tsarevich is typically the hero opposing Koschei, not the sorcerer himself.