I pop up in my fanfics as Maple, Phoenix Stone, and later Hikari. Most of the fanfics here will be based around the 1000 Years storyline
Cores are the source of all magic of the world, every living being has it but not all things know of it. There are three Cores for each species, each of the Three Spirits created one core for every living creature. Without a barrier, the raw magic of the Cores can erode and destroy the physical form of any creature. Said erosion does not happen right away depending on how the barrier was removed. Damaging the barrier allows the raw magic to be used but at a cost, this slowly weakens and kills