The final planet in Metroid Prime 3: Corruption is Phaaze. It is the source of Phazon, the powerful mutagenic substance that drives the central conflict of the Prime trilogy.
After fighting across planets including Norion, Bryyo, and Elysia, Samus follows the Space Pirates to their homeworld and defeats a Phazon-empowered Ridley. The Galactic Federation then uses a captured Leviathan battleship to open a wormhole to Phaaze. This makes the planet the setting for the game's final act.
On Phaaze, Samus battles through the planet's corrupted interior and confronts Dark Samus. She defeats Dark Samus and the corrupted Aurora Unit 313 at the planet's core. Phaaze subsequently explodes, apparently rendering the Phazon spread throughout the galaxy inert and ending the immediate threat.
A common mix-up is Tallon IV, the setting of the first Metroid Prime, or Aether, the world divided between light and dark dimensions in Metroid Prime 2: Echoes. Neither is the last planet visited in Corruption. Zebes is the homeworld central to the original Metroid storyline.