From Halopedia, the Halo wiki
Narcozine gas is installed as a security measure in certain Office of Naval Intelligence bases, like CASTLE Base on Reach, in order to neutralize potential threats to security. AI constructs inside these bases are given broad discretionary powers, with the power to use this gas on anyone who violates protocol and accesses classified information. The AI Araqiel threatened to use this gas against Dr. Cathrine Halsey at the base. In return, Halsey activated the AI's fail-safe function, erasing Araqiel's Riemann matrix.
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