Everything was changed after the departure of Eren (The Unbeatable). This world of immortals used to be a place of peace and harmony.
No one from the other countries dare to rebel against the spiritual realm because It was ruled by Eren. She was the only person who was able to control every element and was the best alchemist in the realm.
In this world their was the law of nature that only the fate can choose the one with immense power and when the spirit of the chosen one will leave their body then the heir of this power will be born again.
Only a person with pure heart can fully control this power, if the soul of the owner start to walk on the dark path then the power itself destroy it's owner.
To hold such power the vessel (body) has to go through under some hardcore training only then the power will start to build. The owner will suffer the pain of dying once in his life.
This power absorbs the previous hier soul and hence the power kept on increasing generations by generations.
The knowledge of all the Masters who used this power is given to the next master. It depends on the person when they can awake the true power.