Assassin

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

The Assassin Medal also known as the Assassination Medal is awarded and achieved for performing an assassination on an opponent from behind. It is available in all multiplayer gametypes. In Halo: Reach, there are two medals that are awarded to the players for meleeing an opponent from behind, which are the Beat Down and Assassin medals.

Description
The Assassin medal is represented by a knife in front of a stylized X. 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)." In Halo: Reach, the Beat Down medal is represented by a ninja on an eight-pointed star, previously called Assassin in Halo 3. It can be acquired when Campaign Scoring is activated and also in multiplayer. There are some occurrences where direct frontal punches result in an assassination, while assassinations sometimes result in beatdowns due to the attacker meleeing to the side.

The requirements for the Assassin medal have been altered for Halo: Reach. If the player melees an opponent from behind, the Beat Down medal is awarded. However, if the player assassinates an opponent through the third-person animation mode, they are awarded the Assassin medal instead of the regular Beat Down medal.

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.
 * Performing seven assassinations in a row without being spotted on any level of Halo 2 Vista unlocks the Silent But Deadly achievement.
 * Performing an assassination against an opponent in a Halo 2 Vista multiplayer match unlocks the Assassin achievement.
 * Performing five assassinations in a Halo 2 Vista multiplayer match unlocks the Ninja achievement.
 * If a player assassinates three players on a Legendary Map Pack map in Halo 3(at either Avalanche, Ghost Town, or Blackout), the achievement "Came...From...Behind" will be unlocked for 50 Gamerscore. Similarly, the achievement can be earned on a Mythic Map Pack map (either Assembly, Orbital, Sandbox, Citadel, Heretic, or Longshore).
 * 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.
 * Earning a Beat Down while jetpacking in a Halo: Reach multiplayer matchmaking game on an Anniverary map unlocks the Dive Bomber 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 What's a Killing Spree? achievement.