Equinox

Equinox: A Journey into the Supernatural is a low budget Cult Classic B-Movie from 1968, which is featured in the Criterion Collection. It is notable because its director Dennis Muren later went on to do effects for Star Wars and was a founding member of ILM.

Recut in 1970 by Jack Harris (the man who also produced The Blob) and re-titled simply Equinox, it enjoyed a long run on late night TV and in the drive in circuit. More footage was shot by a different director, who saw fit to put himself in the movie as the new main antagonist, Asmodeus.

Tropes used in Equinox include:

Bunny Ears Lawyer: Did anyone really believe for a second that Asmodeus was a real park ranger?
Cassandra Truth: No one believes Dave's story.
Character Filibuster: Dave REALLY wants you to know about the mythology behind the book.
Curiosity Killed the Cast
Daylight Horror: The entire movie takes place during the day.
Deal with the Devil: Asmodeus tries to make one to Jim.
Downer Ending
Eldritch Abomination: The tome summons two of them, one, a squid and the other, an ape. There's also an ogre, but it's less eldritch and more fantasy based.
Framing Device: The movie is being told in flashback by Dave in a mental hospital.
Horny Devils: Asmodeus.
In One Year And One Day: YOU WILL BE DEAD!
Kill'Em All: It's implied Dave will die at the end, like all his friends.
Magitech: Believe it or not. The magic in this movie relies on symbols and is described as:

... Manipulation of these symbols is treated exactly like the science of chemistry. This element changes that one, one symbol is a catalyst, another is an agent or a counteractant.

Meaningful Name: Park Ranger ASMODEUS!
Neutral Female: Susan and Vicky.
Screaming Woman: Vicky.
Tome of Eldritch Lore: The book they find which is the crux of the plot.
Too Dumb to Live: "I just remembered! 'Asmodeus' is Greek for 'Devil'!"
What Happened to the Mouse?: After Dr. Watermann dies and disappears from reality, the movie (and characters) seem to completely forget he ever existed at all.