The Katana is a weapon which is added to Fallout 3 in the Mothership Zeta add-on. It was cut from the final version of the add-on and can only be found in the game files.
The sword can be repaired with Chinese Officer's Swords.
When this weapon scores a critical hit, the target will fall to the ground just like the knockdown effect of the Victory Rifle.
This weapon can only be obtained via console commands. It should not be confused with the Samurai's Sword which can actually be found in the add-on.
The Katana appears in the Mothership Zeta add-on for Fallout 3.