There are influences at play that range from Lucio Fulci, Hammer Horror, Re-Animator (1985), The Evil Dead (1981), Basket Case (1982), and a plethora of others.
The key takeaway is that none of this is treated seriously; James Wan is not only freed from the shackles of a PG-13 rating (and the light R-rated fare of his The Conjuring movies), he seems to have no studio notes and has final cut on the project, seemingly making something that would fit right into the video nasty era and receive an X rating during the days of the original The Evil Dead.
However, it throughout draws stylistic influence from all eras and types of horror: it has the suspense and mystery of Psycho and Rosemary’s Baby , the psychological eeriness of Repulsion and Don’t Look Now , and the gruesome imagery of Possession and The Evil Dead .