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If you are referring to the energy sword, a Covenant weapon, it is picked up and used like any other weapon in the game.

If you are referring to the Katana armor permutation in Halo 3, you must "[e]arn 49 Achievements and 1000 Gamerscore (If you only have one or the other, the armor will glitch on and off every other achievement until you have both)" and equip it in the armor customization screen.

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