From Halopedia, the Halo wiki
Bloody Arrow is a UNSC emergency code signifying the complete rout and decisive defeat of friendly forces, usually following the overrunning of friendly units and positions by hostile forces. It is effectively the complete opposite of S.O.S - it means that one should not, under any circumstances, attempt to land, reinforce, or attempt rescue. Rather, it signifies to any reinforcements that the cause is lost and they should turn back.
This code was issued frequently during the Human-Covenant War. A notable instance was during the Onyx Conflict when Sentinels reclaimed that world. When Dr. Halsey and Kelly-087 were entering the atmosphere of Onyx, they received the Bloody Arrow code from Senior Chief Petty Officer Franklin Mendez. Regardless, they continued down and eventually met up with the Senior Chief, Kurt-051, and the remaining Spartan-IIIs.
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