It is a Legendary Sword in which the Eight Headed Snake (or Yamata no Orochi) lives.
Kusanagi Sword is one of the three sacred treasures of Shinto.
Yamata no OrochiYamata no Orochi is a monster of Shinto mythology. It is said that Susanoo slayed the Eight Headed and Eight Tailed Snake that could devour Kushinada-Hime. The Yamata no Orochi was a Serpent with Eight Heads and Eight Tails. It's Length was extended over Eight Mountains and Valleys. It has Trees growing on it's body.
This Sword allows its possessor to control the clouds.