Shichiseiken

The Seven Star Sword is the true antagonist in the fifth movie.

Creation and Revival
The Seven Star Sword was a sword 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 Saga 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 possess a nearby Marine dojo to do its bidding.

Encounter with the Straw Hats and Destruction
One year later, sensing his old friend Roronoa Zoro nearby, the possessed Saga contacts him for its plans to get rid of sacred jewels that could seal it's evil power. However, Monkey D. Luffy manages to find these jewels and the Straw Hat Pirates, 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.

The Sword was initially defeated by Luffy, but then merges with Saga's body, making it near unstoppable. It was even able to disperse Maya's barrier on its 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.

Abilities
The Seven Stars Sword is a powerful blade that has been cursed by the bloodshed of its creation. It grants immense power to its wielder, but corrupts their mind and conquers them. It can also merge with its wielder, giving them the ability to cut objects with their bare hands at any range, becoming nigh unstoppable. The blade is highly durable as Zoro's Yaki Onigiri didn't even leave a mark on it, Zoro himself states that his attack was suppose to destroy the Seven Stars Sword.

Attacks
While Saga was wielding the Sword, he showed off two attacks.
 * Yokazan (妖火斬): Saga launches a green-colored flaming projectile from the sword.
 * Yoja Gashuzan (妖蛇牙重斬): Saga transforms the Seven Stars Sword into a blue-flame snake and attacks with it, in which the snake bites the opponent several times and drains a large amount of blood.

Trivia

 * Chinese swords of the Tang dynasty often bore the mark of seven stars, the Big Dipper constellation, on their blade and are referred to as seven-star swords. This is further supported as the Seven Stars Sword's design is based on a Chinese sword.