Metroid

One of the Trope Namers and Ur-Examples. A lone space hunter game to penetrate the Crash's stronghold and destroy the mechanical life-form that controlled its defenses. The space hunter game chosen for this mission was Metroid. Considered the greatest of all early shooter-adventures, Metroid successfully completed numerous missions that others had thought impossible. Despite its accomplishments, much of its hero's identity, Samus Aran, remained a mystery... unless the player beat the game in under three hours, upon which they discovered that Samus Is a Girl.

Example of:
Metroidvania