Kill from the Grave
From Halopedia, the Halo wiki
Kill from the Grave is a medal awarded in Halo 3, Halo 3: ODST, Halo: Reach, Halo 4, Halo: The Master Chief Collection and Halo 5: Guardians multiplayer, campaign, Firefight and Spartan Ops to a player who kills an opponent after they themselves have died. In Halo 3 and Halo 3: ODST the medal is known as Death from the Grave and in Halo 5: Guardians as From the Grave. As this medal is awarded, a message appears on the screen, saying "You killed (player) from the grave". Unlike earlier games, in Halo 5: Guardians the medal is also announced by the multiplayer announcer.
In the first three games the medal is represented by a platinum eight-pointed star with a face without a mouth in the center of a silver six-pointed star. In Halo 4 and The Master Chief Collection the star is also placed atop a blue circle and grey cross.
The medal most often occurs from a grenade (more commonly Plasma Grenades) or Needler rounds detonating after the player's death, or a slow moving projectile such as a rocket making contact with a foe after the player who shot the rocket is killed. In Halo: Reach it is also possible to earn numerous Kill from the Grave medals with the Target Locator in campaign and Firefight. It appears to occur frequently in Halo 4 in case of near-simultaneous kills (e.g. melee duels or Shotgun/Scattershot duels). In Halo 5: Guardians the medal will be earned in Warzone if player injures another player, dies, and then that other player is finished off by an AI marine, Covenant or Forerunner.