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Mystery House is an adventure game released by On-Line Systems on May 5, 1980. It was designed, written and illustrated by Roberta Williams, and programmed by Ken Williams for the Apple II. Mystery House is the first graphical adventure game and the first game produced by On-Line Systems, the company which would evolve into Sierra On-Line. It is one of the earliest horror video games.

Wikipedia. CC BY-SA 4.0. Updated Jul 21, 2026.