Souls

The souls that exist inside the world were all created by a god/goddess. And the reincarnation cycle is something that can recycle the souls.

When a person is born, a soul is given to the body, and when the body dies, the soul goes back to the reincarnation cycle. The same soul could reincarnate multiple times, but doing so will decrease the 'durability' of the soul, and damage it.

With the goddess of the world gone, and the god who was supposed to be managing the world not really working, no new souls are created. Therefore, the reincarnation cycle is currently filled to the brim with recycled souls, with their durability almost running out.

The collapse of the world can be divided in two different categories, the first being the MA Energy crisis, the second being the Soul Durability crisis.

With all the souls almost running out of durability, and disappear completely when the durability runs out, all life will eventually cease to exist. Even if there is enough energy, with no soul, nothing will truly be alive.

An outsider, named "D" decided that he wanted to add some new souls, and sent 26 souls from a different world into it.