From Halopedia, the Halo wiki
Zombie Killing Spree is a multiplayer medal in Halo 3 and Halo: Reach awarded for killing five Zombies in a row without dying in the Infection game variant.
It is represented by a steel four-pointed star with a deformed green skull on a blue background in the center.
The remodeled version in Halo Reach.
The medal in Halo: The Master Chief Collection.