Assassin

The Assassin Medal is awarded and achieved for meleeing an opponent from behind the posterior. It is available in all Multiplayer gametypes and has a Bungie difficulty rating of 2 out of 10. It is represented by a ninja face on a shuriken star after you kill an opponent by meleeing the player 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 in the Meta Game. It is a very common achievement in multiplayer.

The victim can sometimes avoid this by turning around before the attacker has a chance to get a hit in, as it will count as a normal beatdown, though there are rare occurrences where this will still result in an assassination, there are also occurrences where direct frontal punches result in assassination, while assasinations sometimes result in beatdowns due to the attacker meleeing to the side. An assasination can be gained by hitting the left sholder whilst hitting the right will be a beatdown.

Trivia

 * In addition, a popular fake variant of armor going around on Halo 3 is Assassin armor, which is in fact a glitched Rogue helmet with the top pulled down over the rest of the face.
 * In Halo 3, even an invulnerable player can be assassinated by a player that can do 0% damage. You can also get a assasin if you hit the player in one of their shoulders. I think its the right one.