The Weave is a gigantic construct that determines the actions of everyone, their future choices are all defined by the Weave and led by it. Conversely, the weave is also a record of every existence to ever be.
From the very first to the very last, if there is a last. Everything is held within the Weave, their life and death, however, when a being reaches the stage of 'Mythic', a complete harmony between their physical form, their mental form, and their soul. Something shifts in the Weave.
The Weave, then regards this individual with some importance. It gifts them with a sort of immortality. Anyone who has reached that stage will never die of old age, however, what about being killed?
It doesn't work, to put it simply. Their existence is simply regarded as too important to lose, therefore, they can be labeled as an Endless Being, they who cannot die no matter what.
