Aberzanzorax
What gives them form? The desires of a mortal, who gives the oni created a name (and an envisioned form and purpose).
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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.