From Halopedia, the Halo wiki
The power drain is a combat support device which was used by the Covenant's Jiralhanae forces. It created a short-range electromagnetic pulse (EMP) effect, which interfered with all electronic equipment within the radius of the pulse. It was primarily designed to interfere with common varieties of energy shielding and the electrical components of vehicles. After a short time, the device would overheat and explode, which could wound or kill combatants in its resulting explosion. The electricity it produced could also cause significant harm to unshielded biological targets. Power drains saw usage by Jiralhanae during the last few months of the Human-Covenant War, both during the Battle of Mombasa and the Battle of Installation 00.
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