Commander Keen 3: Keen Must Die!

The trail leads to Vorticon VI, the aliens' home world, where a mysterious Grand Intellect has turned the once-peaceful Vorticons against Earth. Keen fights through the Vorticon cities to the Grand Intellect's fortress to end the invasion — and uncovers a familiar young foe behind it all.

Keen Must Die! is the climactic third episode of Invasion of the Vorticons (Apogee, 1990), available only in the registered version. It concludes id Software's very first commercial game series.