The Horrors act as a type of corrupted divine magic, or un-pure God Magic. This is primarily through means of God's procreating with mortal races and the magical essence of the mortal side overpowering the celestial side causing tainted blood. Another way is an individual killing a God and claiming their powers, or those of far superior magic comprehension mastering multiple schools of magic that rival that of a God.
Horrors are being who often act as a secondary deity and bestow their magic unto beings of their own choosing.
Horrors born to the God's are bestowed a unique mark on their body in red (1st), those who have killed a God have a mark that appears black (2nd). Those who surpass in magic aura have a mark that often appears in a dark purple (3rd). Fate branding them directly as a means of identifying the tainted blood.
Although the Unknown all hail from God's they are not deemed as Horrors. Cana is the only one branded as a Horror due to her overpowering human nature that has diminished the celestial side of her. Because of her human will and drive, it has caused her to be the most revered Saint, but considered inferior when compared to her Horror compatriots.