Hereditary is an awesome horror film that is pretty underrated. If you want to watch Hereditary online then here are a few options (we checked these as recently as July 2019):
Amazon Prime Video
The entire film is available on Amazon Video, and you can sign up for Amazon Prime Video completely in 10 seconds and have access to it immediately, plus a whole bunch of bonus content, and watch it immediately!
Just cancel before the 30 days is up and you don’t pay anything, simple. If you’re wanting to watch Hereditary online and in high quality right now, then a trial with Amazon Prime Video is your best option.
Netflix + VPN
Hereditary is available to watch online through the Australian version of Netflix, and to access this version you could easily use the 30 day trial offered by ExpressVPN.
Here is the trailer for Hereditary: