Hakkai and Kusen are two specially crafted swords handed down in the Kuroda family. Created by the first Stormcloud/Amegumo, they are designed specifically for fighting styles that use superhuman speed. Specifically, although superior speed allows a blade to cut through matter that it would otherwise be unable to, extended use of a sword under this kind of strain results in a vastly increased rate of degradation to the cutting edge and damage to the blade in general. As a result, both Hakkai and Kusen constantly draw on elements in the environment during a battle to reconstruct themselves, although the seals that allow them to do so must be periodically renewed.

The names of the two swords originate from the phrase "Kusen Hakkai", which refers to the belief that the universe consists of nine mountains and eight oceans.

Amusing Things:

Hakkai can also be written to mean "eight times" (something that was used for joking purposes in Gensomaden Saiyuki. "Hakai" is destruction.

"Ku" and "sen" can mean nine and thousand specifically (although nine thousand is actually "kyuusen").