Commander Keen: Invasion of the Vorticons: Episode 1: Marooned on Mars (Dec. 14th 1990) Episode 2: The Earth Explodes (1991) Episode 3: Keen Must Die! (1991) Commander Keen's very first adventure, and the debut of id's groundbreaking side-scrolling technology.
The Four Horsemen of the Apocalypse lurk in the shadows before you. They are Death, Pestilence, Famine, and War. They are the root of all that is evil, but the least of your worries. The last of the three Serpent Riders yet lives.