He's Unstoppable! is an achievement in Halo: Combat Evolved Anniversary and Halo: The Master Chief Collection (in which its name does not include the exclamation point). It is awarded for completing any level on either Heroic or Legendary difficulty without taking any damage to the player's health. It is worth 20 Gamerpoints in the original game; in The Master Chief Collection it is only worth 10 points.
In the original game it is represented by the Legendary difficulty icon with a symbol of a white cross inside a red hexagon on the Sangheili skull; in The Master Chief Collection it is represented by a screenshot of Day of Jubilation's pinch fusion reactor.