From Halopedia, the Halo wiki
The Killtrocity medal is awarded for killing six 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 six bronze stars on a gold circle. In Halo 4 it is represented by six white stars on an olive green circle rimmed with yellow and brown. The medal is monochrome and imposed on a shield, with the stars arranged differently in Halo: Spartan Assault and Halo: Spartan Strike. When a player gets this medal, the announcer will shout “Killtrocity!” to the player that received the medal.