Each world has two layers, one is the outer world, which is the world we are in now; the other is the inner world, a world that we cannot see, maybe ghosts can see... There is a tree in each inner world, called the World Tree, and the World Tree will give birth to a life substitute They are in charge of the balance between the outer and inner worlds. These people are also called "World Trees". Although the lives bred by the World Tree will not die naturally, they can be killed, so these people who are bred will choose a suitable person to become the new World Tree after their unnatural death.
Of course, in the normal world, there is only one World Tree in the inner world, and there is no World Tree in the outer world, so the normal inner and outer worlds have a symbiotic relationship, but there are also special existences. This world is called a "failure". This world is a world composed of an inner world and another outer world, so there are two world trees, one in the inner world and one in the outer world. It is not a symbiotic relationship. The inner world has no impact on the outer world, and the outer world has little impact on the inner world. The story officially begins here.
Purpose: It is everyone's responsibility to care for trees