Shichiseiken

The true antagonist behind the events of movie 5, Curse of the Sacred Sword, it is a sword that was created from the bloodshed of three warring kings who tried to win the heart of a maiden thus absorbing all the negative energy around it. The maiden sacrificed her life to stop the warring kingdom and, in their sorrow, the kings sealed the sword away.

Years later though Saga during a pirate attack, found the sword in a crypt he and his girlfriend, Maya, were cornered in and used it out of desperation to save her. However the sword's evil influence possesses Saga and brainwashes him into trying to rise to full power, using its power to posses a nearby Marine dojo to do its bidding.

A year later, sensing Zoro nearby, the possessed Saga contact him for it's plans to get rid of sacred jewels that could seal it's evil power. However Luffy manages to find these jewels and the Straw Hats, with the help of Maya, are able to enact a ritual that will suppress it power. The sword tries to stop this and nearly succeeds.

It was initially beaten by Luffy, but then merged with Saga's body, making it near unstoppable. It was even able to disperse Maya's barrier on it own. However, it was finally stopped when Zoro appears and calls to Saga's consciousness, challenging him to a sword duel. Zoro manages to slash at Saga but only destroys the sword's influence allowing Saga to live.