Aristocrat said: "Front Oni" and "Back Oni"? What's referenced here?
Zenki and Gouki (前鬼 and 後鬼, in the lower left circles as you saw) are a famous pair of oni, husband and wife, that served En no Ozunu, the historical founder of Shugendou, as shikigami. A pair of bird shikigami with the same names (whether they were intended to be the historical figures is unclear; the ability to change forms is common for shikigami) also appeared in Silent Sinner in Blue serving Yukari.
Edit: Or I guess Moonspeaker already linked most of that. Oh, well.
Hey, I didn't know about the SSiB angle, so it wasn't in my links, plus some readers don't like linking out to get info, so I don't think you wasted your time.