Spirits are divine beings that enter into a covenant with Guardians in order to live in the material world. When a Spirit's guardian dies, the Spirit returns to their celestial city called Galaxia where they wait again for the Summoning.
However, there are extremely rare cases where a Spirit without a guardian remains in the material world, these Spirits will eventually turn into a Void. They feed from the darkness of humans in order to survive and live. They cause chaos, mayhem, and destruction. Voids, therefore, are very dangerous beings.
Voids can only be saved through a Purification that can only be done by guardians. It is the counterpart of ending a life. In other words, to purify a Void means to kill it. This is the only way to save them. It is a guardian's duty to save them.