Suppose humanity (or something like it) lives on a planet where we have discovered the existence of Souls. Each time someone is born, a new soul is created, but this is a maximum number of souls to go around (I’ll put it at about 6.5 billion, which is a number that would easily be supported by this planet) every soul belongs to a “nation” which is the land area that would be considered the “respawn point” for a soul. A soul will cycle between an animal (with human-level intelligence) and a human body. However, the behavior and skills of someone in their human life can influence what they become.
Soul’s new bodies are given a new body by guardians (who can see the most recent life of a soul by looking at it). These guardians can also deny a soul resurrection, but rarely do (usually mass-murderers etc.). Humans emerge at the age of about 16, with the skills and knowledge of their smartest life but the memories of their past two lives. Animal resurrections can be any creature, mythical or real.
There are a few ways to kill a soul for good (though a new soul will be added to the pool of “possible souls”).
Be denied reincarnation by the guardians.
Have the heart of your body eaten by another soul (animal or human) before it's buried
Soulkillium is a metal that can kill a soul, it is very very rare and takes a large quantity in one place to work(3+kg).
Using one's soul to attack someone else's. (Kinda like black star from soul eater.)
What sort of changes would we see to say classical medieval society. In specific interaction between nations and the whole feudal system.