In a wider sense a game called Dungeon my claim to be the first FPS. The player had to explore a cave filled with treausers, terrible enenies like dwarfs, orcs (?) and other. But Dungeon was not a video game. You moved and acted via the keyboard on a dialogue screen. However, it draw the interest of many students (and has cost many hours sleep) in the mid resp. the end of the seventies at the universities, where it ran on DEC PDP11/xx under RSX. I remember well computer rooms wallpapered with the reconstructed topology of the caves.