Katana

The Katana (sometimes also incorrectly called the Hayabusa sword) is a special Armor Permutation unlockable by earning achievements  (1000 gamerscore in Halo 3 out of 1750, and at least 49 Achievements. Halo 3 has 79 Achievements in total, and all skulls (including silver skulls). The Katana closely resembles Ryu Hayabusa's Dragon Sword, and is not usable by the player.  It has been observed that completing the "new" achievements, thereby boosting gamerscore above 1000, will unlock the Katana or Security helmet (I.e. 990 and then unlocking the new "Double Double" achievement for an end result of 1015 will unlock the Katana). However, you must have unlocked all of the offline achievements.

Characteristics
The Katana consists of a normal Hayabusa body armor with a Katana sword affixed to the back with a white attachment, possibly magnetized. The sheath of the katana is grey and the handle is a reddish brown color.

Trivia

 * There also seem to be some markings on the hilt of the sword resembling Japanese characters.
 * The Katana resembles the first Dragon Sword Ryu gets in Ninja Gaiden, not the True Dragon Sword.
 * The Katana and the Security armor permutations both come with getting 1000 GS/49 Achievements in Halo 3. Because of this, some fans thought that the Katana should have gone with the Security armor instead of the Hayabusa.
 * It is considered by some to be the hardest armor to unlock, along with the Security helmet, because of the difficulty of some of the game's achievements.
 * There are rumors that a player can be damaged "through" the Katana because it extends out from the player's back; however, these rumors were proven to be incorrect.
 * The Sword's color cannot be changed, so it is always gray with a red hilt, and a white armor attachment.
 * Since the Katana is already slung over the right shoulder, the player can no longer use this shoulder for weapons. Instead they use the left shoulder. It has also been observed that by slinging a Sniper Rifle over one's shoulder with the Katana, an X can be formed.
 * Oddly enough, if a player is carrying a back mounted weapon, the gun is slung under the Katana, and where the Katana joins the body, instead of over it. It would seem that it would obstruct the drawing of a weapon, or cause the Katana to fall off, but it doesn't, apparently.
 * It is most popularly viewed with the Hayabusa Helmet and the Scout Helmet.
 * The Katana is now much easier to get due to the new achievements.
 * It is possible to get the Katana without Xbox live if you get all the Campaign Achievements.