Watched this last week. It reminded me a little of The Babadook but instead of being about depression it's about sexual abuse. And also depression. Definitely more of an atmosphere film, but boy what an atmosphere.
Here's some words copied from my letterboxd review:
Feverish nightmare blending into grim reality. Possum is dished out in such a trance-like depressive intensity that the ending knocks you stiff, punching you with the horrors it tickles at throughout. The perennial droop of Sean Harris' face ties the humanity of this film together, which only serves to augment the cruelty of the way he almost seems to merge with his haunting puppet over the course of the film. The puppet's symbolism, by the way, is evident - unexorcisable, unrepressable trauma - but the niggling terror and bleakness in which it is delivered give it a true edge.
I simultaneously did and did not want a hug after seeing this. Highly distressing - and very, very good.