Assassin

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

The Assassin Medal (Beat Down in Halo: Reach) 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.

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. An assassination can be gained by hitting the right shoulder whilst hitting the left will be a beatdown and not an insta-kill. In Halo: Reach the Assassin Medal is awarded only if you melee them from behind and go into the third person animation. Your screen will say "You Beat Down [players name]" when you don't go into third person.

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.
 * There are three categories for assassinations: on ground, on air, and a mix of both. Meaning, there's a specific animation for the attacker to assassinate a victim while both being on ground, while both are on air like with jetpacks, and the last one when a attacker jumps on a victim who's on the ground, each style unique to each species of Spartans and Elites in multiplayer
 * Third-person assassinations can be done to Jackals and Grunts in Halo: Reach.
 * It appears that Elites stab enemies with a smaller energy knife that comes out from the wrist, while Spartans just take out their knives instead.
 * In Halo: Reach  there are special third person Assassinations with the Energy Sword.
 * In Halo: Reach there are no third person assassinations for Skirmishers.
 * The normal Assassination medal from Halo 3 is the new "beat down" medal, and the third-person assassinations reward you with a new, separate medal.