So I am writing up Appendix V, the textier undead/unlife guide and I get a V-8 sort of moment.
In earlier discussions people had been asking about keying an avatar or demon to the material planes and what the problems were with this.
I completely spaced, but this is literally the problem of Specters.
Specters are ghosts/poltergeists that can, for short periods of time, create and maintain anima bodies on the material plane attached to their aethereal plane.
The limit basically comes down to how much mana they have.
One big problem for ghosts in generating mana is that they aren’t in the physical world so “generating mana by interacting with the elements and their animus” doesn’t work so well. They are on the aethereal plane.
Now, if they can get enough mana together they (advanced ones) can in theory create an anima body just like an avatar or demon does when they don’t fully incarnate.