Chosen Ones (The Chosen Ones, #1)

by Veronica Roth

The Needle of Koschei.

Koschei the Deathless is a figure in Slavic folklore-- an immortal antagonist who hides his soul inside nested objects (a needle inside an egg inside a duck, or a rabbit, etc.) I wasn't aware of him before writing this book, but I discovered this folkloric figure while looking into folk tales that have specific objects attached to them.