From Halopedia, the Halo wiki
Retrieved from "https://www.halopedia.org/index.php?title=Assassin&oldid=1333841"
The Assassin medal (Assassination in Halo 4 and Halo 5: Guardians) is awarded for performing an assassination on an opponent from behind in Halo: Reach, Halo 4 and Halo 5: Guardians multiplayer. It is also awarded in campaign and Firefight in Halo: Reach, and in Spartan Ops in Halo 4.
The Assassin medal is represented by a knife on a circle in front of a stylized X. In Halo: Reach the circle is red; in Halo 4 it is blue. 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)."