Assassin

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

The Assassin Medal is awarded and achieved for meleeing an opponent from behind. It is available in all multiplayer gametypes and has a Bungie difficulty rating of 2 out of 10. In Halo: Reach, there are two medals that are rewarded to the players for meleeing an opponent from behind, which are the Beat Down and Assassin medals.

Description
It is represented by a ninja face on a eight-pointed star. After the player kills an opponent by meleeing the opponent in the back, a message appears on your screen that states: "You assassinated (player's name)". The Assassin Medal is also a medal in the Halo 3 campaign when Campaign Scoring is turned on and activated. There are some occurrences where direct frontal punches results in an assassination, while assassinations sometimes result in beatdowns due to the attacker meleeing to the side.

The medal was renamed to Beat Down in Halo: Reach for meleeing an opponent from behind. However, if the player assassinates an opponent through the third-person animation mode, they are rewarded the Assassin Medal instead of the regular Beat Down medal.

Trivia

 * If a player assassinates three players on a Legendary Map Pack map, 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.
 * In Halo: Reach, what was a "beat down" 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 behind a person and going into third person for an intricate death.
 * The achievement, "That's A Knife", is awarded when a player gains this medal whether in Campaign, Firefight, or Matchmaking.