This article might have further canon details available on.
Spirit oni are the least of all oni. Unlike their more powerful kin, they do not manifest their own humanoid form, but are bound into ugly masks by evil spellcasters who force them to serve as familiars.1
When an evil spellcaster wishes for an oni as a familiar, they employ forbidden blood pacts to lure a disembodied oni spirit into a specifically-crafted oni mask, transforming it into a spirit oni—an oni made flesh directly from its spiritual form, instead of taking on a humanoid form. A few travellers to the Forest of Spirits have mastered this process and spread it to other nations.23
Spirit oni bound to a master cannot take humanoid form. As a result, they are foul-tempered and cantankerous creatures, even to their masters, who must regularly reprimand and discipline them to keep their envy and bitterness in check.2
A spirit oni gains free will when its master dies, but is usually too cowardly and dishonourable to take its own life so it can return to true spirit form and then become a proper oni. Instead, such masterless spirit oni seek out more powerful oni to offer their services, in the hope that they might someday be promoted from their lowly state to something more befitting their egos and desires.2
- Mike Shel. (2011). Ecology of the Oni. The Brinewall Legacy, p. 73. Paizo Publishing, LLC. ISBN 978-1-60125-361-3
- Jesse Benner et al. (2011). Bestiary 3 (First Edition), p. 209. Paizo Publishing, LLC. ISBN 978-1-60125-378-1
- Alexander Augunas, Thurston Hillman, Isabelle Lee, Stephen Rowe, and Christopher Wasko. (2016). Haunted Heroes Handbook, p. 4. Paizo Inc. ISBN 978-1-60125-884-7