Mother Brain is the main antagonist of Phantasy Star 2 (Sega Genesis). It is a computer system designed to transform the deserts of Motavia into a green, habitable land, but has been corrupted by an unknown destructive force.
When Earth became uninhabitable, several humans built the spaceship "Noah" and escaped into space. For years they wandered the universe until the discovered the Algol Star System. The decided they wanted it, but they needed to get rid of the native population first. Mother Brain was created in order to fulfill that purpose.