From Halopedia, the Halo wiki
The Killpocalypse medal is awarded for killing nine opponents within four seconds of each other in standard multiplayer games (and Halo: Spartan Assault) and within less time in campaign and Firefight. In Halo 4 multiplayer and Spartan Ops the kills need only be within 4.5 seconds of each other.
Originally, the medal was represented by nine pale pink stars on a red circle. In Halo 4 the it is represented by nine white stars on a brown circle rimmed with reddish-orange and brown. In Halo: Spartan Assault and Halo: Spartan Strike the medal is monochrome and imposed on a shield, with one large star and several smaller stars around it.
This is a very difficult medal to achieve in gametypes other than Grifball.