The first action-adventure game is sweet.
The plot is very simple: a wizard stole the chalice, you must rescue it and slay the dragon. The graphics are extremely plain: squares with some keys and a blinking chalice (which also made all other objects blink when they come in contact with it).
Learning this game is pretty easy: you move. Come in contact with an object and you control it (until you let it go or get another object).
Overall the game is fun, and it started a whole genre. Which makes it revolutionary.