Katana

The Katana (sometimes also incorrectly called the Hayabusa sword) is a special Armor Permutation unlockable by earning achievements 49 out of 79 Achievements.

Characteristics
The Katana consists of a normal Hayabusa body 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. It is not possible to use this item during gameplay.

Disadvantages
Users have reported that like the Security Shoulders, the Katana can re-lock and unlock again. For example, having 980 gamerscore and obtaining an achievement worth 25 gamerscore may unlock them, but they can later re-lock themselves, requiring the player to obtain more achievements in order to unlock it again.

The way the Katana sticks up can reveal you if you are sniping however if it is shot you do not take damage. It also cannot be stuck by a grenade.

Trivia

 * The Katana closely resembles Ryu Hayabusa's Dragon Sword, and is not usable by the player. It was possible to obtain the Katana by gaining 1000 gamerscore. In some cases after the 1.1 update to Halo 3 some people have been able to unlock the Katana by passing the 1000G mark.(i.e. 990 and then unlocking the new "Double Double" achievement for an end result of 1015 will unlock the Katana).
 * The Katana can no longer be unlocked by just earning 1000G in Halo 3. One of the only ways to unlock the Katana after the 1.2 update and 1.3 is to unlock 49 out of the 79 achievements. Unlocking 40 of the 79 achievements worth 1000G, the conclusion is that you MUST have 49 achievements to unlock the Katana. The Katana, as well as the Security permutations, are fabled "armor" glitches, in which they appear and disappear randomly. With the Security, a 1000 gamerscore locks it in, and for Katana, 1750 guarantees it.
 * Since the Katana is already slung over the right shoulder, the player equipped with the sword 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 is formed.