From Halopedia, the Halo wiki
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)."
- Starting in Halo: Reach and in subsequent games, what was a "beat down" in Halo 3 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 and going into third person for an intricate death.
- The Halo Reach achievement That's A Knife is awarded when a player gains this medal in Campaign, Firefight, or Matchmaking. The phrase is a reference to the film Crocodile Dundee.
- Performing an Assassination on an Elite to survive a fall that would've been fatal in the Halo: Reach campaign unlocks the If They Came to Hear Me Beg achievement.
- Performing an Assassination on a player that has just exited Armor Lock in a Halo: Reach multiplayer matchmaking game on a Defiant map unlocks the Paper Beats Rock achievement.
- Assassinating a Promethean Knight in Halo 4 Spartan Ops will unlock the Knight in White Assassination achievement.