A being that unfolded battles, and in the end should have been killed by the Hero.
That body certainly decayed away and returned to darkness.
However, now, after a long period of time, it achieved resurrection.
What accomplished that was The Law of Transmigration.
A magic completed by Bertolt, which converts the body's composition, memories, and soul by binding them together into information, sends that information into the future, and based on that information emulates a body through spirit particles and reconstructs it a reincarnation magic.
Spirit particles are an almighty substance capable of imitating all phenomena, and magic is a thaumaturgy that manipulates those spirit particles, twisting and rewriting the principles of this world.
Theoretically, there exists nothing that magic cannot do. Resurrection of the dead, reversal of time, creation of the universe... if there are the proper formula and magical power befitting it, no matter how absurd, realization is possible.
Reincarnation is also one of those.
Although Bertolt was completing that theory, it was a forbidden art existing only on paper with no successful examples.
Bertolt is a demon race member. The demon race are, without exception, immortal existences. And immortality is an existence that transcends the concept of death.
However, the soul wears down. Even if the body is immortal, the soul is not indestructible.
If the soul is worn away and burns out, eventually it perishes.