Sword of Destruction: Kusanagi

Sword of Destruction: Kusanagi is a Legacy Weapon that was stolen from the Magic Guild by Sechs.

Information
Sword of Destruction: Kusanagi is the most powerful of the Legacy Weapons. It has the power to nullify any Phenomenon Intervention. In addition, the wielder of the Kusanagi gets stronger with each Intervention it nullifies. Thanks to this ability, Sechs retains his memories every time Hinata Himezuru resets the timeline using the Embryo. However, despite its incredible power, Kusanagi is incredibly difficult to actually wield. It requires an immense amount of magical power to activate for even short periods of time, something very few people actually possess. Additionally, being a series of eight large swords, the weapon is unwieldy. Sechs is able to get around this problem with the use of Gravity magic.

Kusanagi's price is “shared existence,” which means that the wielder exists in all timelines simultaneously. Therefore, any injuries sustained in one timeline will affect every other timeline. This ultimately led to Sechs's downfall as Es used the power of the Embryo to send Touya Kagari back to the first iteration of the world before it had ever been recreated to kill Sechs when he was at his weakest. When Touya destroyed Sechs's crystal, the version of Sechs in the current timeline died as well. Additionally, the wielder's soul becomes bound to the Kusanagi, meaning it will never return to the seithr.

Kusanagi fell into the Wadatsumi Gate alongside Sechs when he died. Its fate after that is unknown.

XBlaze Lost: Memories TIPS Entry

 * Kusanagi

''A multi-bladed weapon created using lost technology. Officially known as Sword of Destruction: Kusanagi, it requires a level of magical ability that very few posses and cannot be generally used for any real length of time. It has the power of “phenomenon intervention” and is capable of nullifying the interventions of other beings.''

Etymology
Kusanagi comes from the Kusanagi-no-Tsurugi, which is the original name of the Ame-no-Murakumo-no-Tsurugi. Literally, Kusanagi means "cutting grass".