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anonymousadventure horror film

I think you need to decide which genre it is.

If it's both.

A horror film about a mysterious pyramid in Indonesia.

Most, if not all, horror films have adventures, but not all adventure films have horror.

How about using a synonym of horror, so as to highlight the adventure element of the plot/ story?

A terrifying adventure centred around a mysterious Indonesian pyramid...

I think it’s fair to assume that this is something akin to log-line for a film screenplay, therefore the word film is rather needless. One, because most sites that accept screenplays ascertain wether or not the script is for a film or tv series before even reading the log-line. Two, the importance of using as fewer words as possible for a log-line is basically a prerequisite in the industry.