The only two half-humans in the past 1000 years to posses and to a certain extent, control, a pravus eye are Bladearm and Onii-Chan . Onii-chan had a natural affinity for the eye, and it was far more used to his body than Bladarm's. For this reason Onii-chan was far more powerful. But the eye was too comfortable. Instead of leaving the body when it's full power was unleashed, it instead devoured the human body and dismambered what was remaining.
Bladearm's eye is much more powerful, but his body limited his ability, until he gained his brother's arm. His brother's arm acted as a conduit for his latent power. Commanding the eye became second nature, and most of Bladearm's powers are derived from it's near eternal energy reserves.
It is said that the eye is a warning of times to come. Once the demons have possesed enough energy to be able to turn a human fetus into a demi-demon, it means that they will soon have enough energy to walk the earth again.
It is unsure if this legend is true, or if the pravus eye is in fact a curse or just a biological anomoly. But the church continues to fight against this unseen foe by killing children who exibit any demonic features. They have been unsuccessful.