From Halopedia, the Halo wiki
Cluster Luck is a multiplayer medal available in Halo 5: Guardians and Halo Infinite. It is awarded for killing 2 or more enemies with one grenade blast.
The Cluster luck medal as it appears in Halo 5.
The medal in Halo Infinite seen in the Technical Previews.
The medal in Halo Infinite.