In my fantasy world, certain people have the chance to consume a certain essence that gives them magical abilities when consumed. This essence gives them the ability to insert a piece of their "spirit" into a dead body. This reanimates the body into the wielder's control. This can be done with multiple bodies at a time, and they become a kind of fantasy hive-mind.
However, this would understandably have many negative effects on the wielder mind, having multiple bodies to control is not what it is built for. Think multiple perspectives of sight, touch taste, etc.
So, how would one prepare themselves for such a task? Remeber that this is a fantasy medieval world, where the people we are talking about have the technology equivalent to that of Imperial China.