Assassin
From Halopedia, the Halo wiki
Summary
The Assassin Medal is awarded for meleeing an opponent from behind. It is availible in all Multiplayer gametypes and has a Bungie difficulty rating of 2 (out of 10). It is represented by a ninja face on a throwing star. After you kill an opponent by meleeing him in the back a message appears on your screen that says: "You assassinated (player)".
