Humbler stun device
From Halopedia, the Halo wiki
The humbler stun device, often called a "shock stick", is a purple or blue colored, non-lethal electrical baton weapon used by the UNSC Marine Corps and law enforcement agencies mostly for "motivating" and/or subduing others in a temporarily painful way, but with no long-lasting effects. The humbler temporarily shorts out its victim's nervous system, generally resulting in loss of voluntary urinary functions.
When the SPARTAN-II program candidates first arrived at Reach in 2517, the batons were used by Chief Petty Officer Mendez to "motivate" them at first, though they soon grew to cooperate with him and the devices were not needed.
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