Archon

Oni are Transapients in a state of ignobility. They are a product of either faustery of Sapients or Kami, or tripping of Kami.

Metaphorical use
As with the term Kami the term Oni is often used metaphorically in the 21st century to refer to powerful humans in a state of ignobility with villainous or at least morally ambiguous implications. The reasoning behind this is that until Transapients properly come into existence in Terragen civilization these humans are the closest thing to Oni, and they can help us understand Oni when they actually come into existence