It is indeed a very intriguing film. But even an outline of the story would still be rather disturbing! I won’t have a lot of time to write any time soon, but the Wikipedia entry does a good job explaining the plot (in a cold, indifferent manner, which suits a certain overarching theme quite well). You mentioned your imagination being worse than movies, but for me, this was the creepiest aspect of it; it felt like a story that was being pulled directly from my subconscious to scare me specifically as much as possible.
The other day, I saw that the film recently became available on iTunes and was like, “Oh no! I can’t escape this!” I’m not sure if I’ll have the courage to watch Hereditary again, but I know the director is working on his second film, which I will surely see in theaters. I’ll watch each of his films at least once!