Narcozine gas
From Halopedia, the Halo wiki
Narcozine Gas is a chemical agent used as a weapon by the UNSC. When inhaled, the gas quickly causes paralysis, and extended exposure causes Pulmonary edema, swelling of the lungs, which can quickly become fatal.
The UNSC doesn't actually use the agent in battle, but it is instead installed as a security measure in ONI bases, like CASTLE Base in order to neutralize potential threats to secuity. AIs inside these bases are given broad disgresionary powers, with the power to use this gas on any humans who violate protocol and access classified information.
