Source: Bestiary 3, pg(s). 209
A spirit oni resembles an evil-looking animated mask shows the face of a horned fiend, its eyes popping and its leering smile full of fangs.
Most oni that emerge on the Material Plane do so into a new body of humanoid flesh, yet this is not the case for all oni. Periodically, when an evil spellcaster wishes to engage the servitude of an oni as a familiar, the result is something relatively unusual—an oni made flesh directly from its spiritual form rather than one who takes the features of a humanoid as its own. The ritual to gain such a familiar requires the spellcaster to craft a particularly fiendish and fierce-looking oni mask. When the ritual is complete a raw oni spirit is lured into the mask, which immediately transforms it into the animate, living creature called a spirit oni.
While the ritual binds the oni to its new master, it also robs the oni of its immediate chance to achieve a human form. As a result, spirit oni are foul-tempered and cantankerous creatures, even to their masters. They must be regularly reprimanded and disciplined, lest their jealousy and bitterness get the better of them.
When a spirit oni's master dies, the spirit oni gains free will and the ability to make its own choices—but the spirit oni's innate cowardice and dishonour usually prevents it from taking its own life in hopes of returning to true spirit form and then returning as a proper oni. Rather than extinguishing themselves, such masterless spirit oni seek out more powerful oni and offer their services to their superiors, in hopes that they might someday be granted a promotion from their lowly state to something more befitting their egos and desires.