It's about a man who is essentially immortal; his soul is separated from his body and hidden inside various animate and inanimate objects and animals. However, if anyone has hold of any of the objects, he's entirely at their will and loses his magical abilities.

Koschei would also menace young women and may have been a prince.

((ooc: MOSTLY UNRELATED BUT basically, 'Koschei' is the nearest canonical (it's novel!verse, okay) name in existence for bb!Master young!Master and considering everything he does has two purposes: survival and humiliating the Doctor it's ... loosely relevant.))