From Halopedia, the Halo wiki
When used, all hostiles and allies in the area will be temporarily stunned, for approximately four seconds. After each use, it takes around five seconds to cool down and recharge. The stun blast is particularly useful when surrounded by enemies. Players must use the stun blast at least once, to earn the Powered by MJOLNIR achievement. In Halo: Spartan Strike it is replaced by the largely-identical stun grenade.
List of appearances