Expect extreme blood and gore, with lots of scary stuff, deaths, stabbing, slicing, and brutal insanity. There are also references to upsetting real-life events going on at the time (1977 Germany), including bombings and people being shot in the head.
Dario Argento's Suspiria (1977) is a classic. There's simply no other way to describe it. Its use of garishly bright color, the amazing soundtrack by Argento's band Goblin, and Daria Nicoldi (Argento's wife at the time)'s fairy-tale-inspired story are groundbreaking.