Were you looking for the Halo 2 Vista achievement of the same name?

The Assassin medal (Assassination in Halo 4) is awarded for performing an assassination on an opponent from behind in Halo: Reach and Halo 4 multiplayer. It is also awarded in campaign and Firefight in Reach, and in Spartan Ops in Halo 4.

The Assassin medal is represented by a knife on a circle in front of a stylized X. In Halo: Reach the circle is red; in Halo 4 it is blue. After the player kills an opponent by performing an assassination on an opponent from behind, a message appears on-screen that states: "You assassinated (player's name)."

Trivia

• In Halo: Reach, what was a "beat down" in Halo 3 is now a "pummel." What was an assassination (or a melee in the back) is now a "beat down," and an "assassination" is now holding the melee button and going into third person for an intricate death.
• The Halo Reach achievement That's A Knife is awarded when a player gains this medal in Campaign, Firefight, or Matchmaking. The phrase is a reference to the film Crocodile Dundee.
• Performing an Assassination on an Elite to survive a fall that would've been fatal in the Halo: Reach campaign unlocks the If They Came to Hear Me Beg achievement.
• Performing an Assassination on a player that has just exited Armor Lock in a Halo: Reach multiplayer matchmaking game on a Defiant map unlocks the Paper Beats Rock achievement.
• Assassinating a Promethean Knight in Halo 4 Spartan Ops will unlock the Knight in White Assassination achievement.

Gallery

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  The Assassin medal in Halo: Reach.