Skunked!

Skunked is a Multiplayer Achievement in Halo: Reach. This achievement is awarded when a player wins a game of Invasion in the first phase. It also unlocks the Invasion Nameplate in Halo: The Master Chief Collection.

Walkthrough
Winning a game in the first phase is only possible while defending in a standard Invasion match, not Invasion Slayer. To unlock the achievements players must successfully defend the two marked territories in phase one on Boneyard, Breakpoint or Spire.

Trivia

 * The only way to earn this is to play as the defending team in Invasion (for instance, the Spartans on Boneyard) and stop the attacking team before they complete the first phase objective.
 * The title of the achievement refers to a sports rule where one team has an insurmountable lead over the other team.
 * The announcer says this if the Spartans prevent the Elites from completing the first phase on Breakpoint.