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Builder Security was a Builder military organization. They conducted security and combat matters for other Builders; in many cases, they effectively served as the Master Builder's private army. After the Ur-Didact went into exile for the first time, Builder Security almost entirely supplanted the Warrior-Servant rate, with most former Warrior-Servants joining their ranks after their own rate was diminished. However, many former Warriors retained their loyalties to their original rate. They took part in defense of large scale evacuations of Forerunner planets, making use of the Z-180 Close Combat Rifle in close quarter combat against the Flood.