Demons

Demons are a type of Fiend whose origins remain a mystery. All that is known of Demons is that they are invasive, chaotic creatures which have in insatiable hunger for Mortal souls. Consumption of souls allow the demon to grow in strength, and to split into multiple Demons. It is unknown what governs whether a Demon grows in strength or reproduces, but they remain a great threat to Mortals.
A third, very rare outcome is possible when Demons consume a Mortal's soul. If the soul is in some way, unable to be destroyed by the Demon, the Demon is instead destroyed, and a Pure-Blood is created instead. There are no known ways to shield a Mortal soul to produce such results.