and...Although hmm. I'm still thinking of these as less a species, and more a supernatural result (I can't think of them as breeding). Sort of like how the Wendigo legend is related what happens when a man goes cannibal. Perhaps a cautionary tale of what happens when a person casts aside everything to pursue a single purpose; losing yourself in your calling and being consumed by it, resulting in the loss of your humanity and Wickedness/Other taking up residence in the emptiness you have left.
Aberzanzorax
Oni come into being out of an extreme desire...a sort of deal with a devil. They are wisps of negative emotions and raw, untapped power until given form.
Aberzanzorax
What gives them form? The desires of a mortal, who gives the oni created a name (and an envisioned form and purpose).
<snip>
The oni will seek, at first, to fulfil its purpose for the master. Over time, however, the purpose becomes all consuming. The vengeance Oni lives to revenge the tiniest slight...Oni quickly become consumed, each with its own particular type of extreme eccentricity. Their purpose utterly drives them to the point that the tiniest slight that negatively affects their purpose results in an extreme (and usually deadly) response.
The problem is that usually, the master, whose name the oni has been given, inevitably commits the sin that they so hated (fickle and imperfect as humans, but not oni, are). When the master is killed, the oni is free to concoct schemes, becoming proactive (the betrayed lover sets up a brothel and kills married johns) rather than reactive.
I wouldn't mind seeing an interesting plot hook / adventure seed involving giant eagles.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.