Nephilisme

Nephilim (Half Angels)
 
These beings are strictly forbidden. Angels are barren with each other and cannot produce their own children with other angels. Likewise, they can not produce offspring with any monster or mythical beast. The only one who can procreate with an angel is a human.  This creates a Nephilim child, half angel, half human. It is beyond taboo. It is a forbidden, horrific thing to angels. Not only because it required an angel to procreate in the first place, but also because of the cruelty to the child and its human parent.
Nephilim are highly unstable. Their human half is incompatible to their angel half. They often do not even survive to or past birth. In the rare chance that they do though, they will not live long lives on their own. Most do not make it to five years of age. None make it past puberty.  Their angel side will burn out and destroy the human side and, at the same time, the human side will corrupt and tear apart the angel side with its fallible nature. They will either die or turn into a demon, neither one more common than the other.
This same corruption also means that, like angels, they can easily fall into a spiral of evil, their corruption breeding more and more corruption. Nephilim can live as idyllic children with what appear to be mild to severe illness…or they can become the darkest, most evil people possible on the Earth, seeking only to satisfy their own lust for hurting others and satisfying their every whim. It is a cruel fate and often ends in tragedy.
The only way to prevent this fate is to burn away the corruption, the same as with any other angel being. The child, preferably young, must be brought to the Hearth and undergo the process of burning off their sin. This, however, will not leave them the same as they came. The Hearth burns ALL that is not of angel origin. Their humanity and human form will also be completely destroyed, leaving only a now pure angel behind, aging them into an adult form. Their level of power depends on their parent and on their individual soul, but none exceeds the range of normal, angel ability.

The families of Nephilim that become corrupted are greatly effected. Even though no Nephilim gets old enough to have a child of their own, their corrupted essence carries on. Any blood relative they have will suffer misfortune, bad luck, or generally begin to have dark thoughts or actions themselves. The closer the blood relation, the stronger the effect. Families of Nephilim have notoriously bad luck because of this.

(You /can/ make a Nephilim child, but it must be a young child. 5 or less. This is because there is no way a Nephilim will remain uncorrupted longer than that. By that point, even, they're probably on their last legs. So, if you make a child, keep that in mind. Also, they'll have to be purified into a full angel in the hearth if you so decide to age them up. If they get turned into a demon, they'd be killed, so it's a tricky game.)