Seven Warlords of the Sea

The Shichibukai (七武海 Shichibukai, literally Seven Military Seas, The Seven Warlords of the Sea in the English versions) are a set of fictional characters in the anime and manga series One Piece.

About the Shichibukai
The Shichibukai are seven of the strongest pirates in the world. The Shichibukai is an organization of seven privateers that are in the service of the World Government. These seven individuals are former pirates that hunt active pirates, though they apparently share a percentage of their income collected from this bounty hunting with the World Government.

The Shichibukai are apparently granted a great deal of autonomy in how they operate from what has been shown to date. Crocodile seems to have been unsupervised for an extremely long period of time and several high-ranking Marines expressed surprise that several of the Shichibukai actually bothered to show up when summoned for a meeting. Presumably the World Government tolerates these actions from the Shichibukai because of the risk involved with maintaining the three great powers.

Because of their deal with the World Government, the Shichibukai are considered "Government Dogs" by other pirates. At the same time, they are also feared by them because all of the Shichibukai are insanely strong

The other known members of the Shichibukai are Donquixote Doflamingo, Dracule "Hawk-Eye" Mihawk, Bartholomew Kuma, Jimbei, Gecko Moria, and most recently Marshall D. Teach. So far there is one member left that is yet to be introduced, plus Jimbei, who hasn't made a physical appearance. Sir Crocodile was initially a member of the Shichibukai, but he was defeated by Monkey D. Luffy at the end of the Alabasta arc, arrested for his crimes, and stripped of his rank. He has been replaced with Blackbeard.

All current and former known members of the Shichibukai have animal-themed names.

How someone becomes a Shichibukai is not known, a person bounty may very well not relevant in choosing a member, Crocodile's bounty is far lower than that of the other know shichibukai and there are people with bounty's higher than his yet he was still chosen to become one, indeed Blackbeard didn't even have a bounty yet he is still given the title

It is also unknown if the former 7 shichibukai are the original or a new group, or if they were all chosen at the same time or more were asked to join as time went on.

The newest Shichibukai so far is Blackbeard.

Duties of the Shichibukai
The Shichibukai are one of the Three Great Powers that help maintain the balance of the Grand Line, though they seem to be a mixture of the more chaotic Yonkou and the more orderly Marines. They do not offer open opposition to the rule of the World Government, but are not guaranteed to follow its orders either.

They made a deal with the World Government for the Marines to drop the bounties on their heads, provided that they work in association with the government. They are only allowed to attack other pirates and must also pay the Government a tenth of their loot. Each member of the Shichibukai must also have strong intimidation skills and a fearsome representation to put off others becoming a pirates and make them fear them.

Their particular goal (or rather the World Government's goal for them) is to maintain a balance with the Marines and prevent the Yonkou from gaining the upper hand in the New World. To this end, they are to be regarded as allies of the Marines and as enemies of the Yonkou, although judging by the those instances where the Shichibukai have interacted with one of the other Powers, they seem to be on closer terms with the Yonkou than the Marines, whom several of the Shichibukai appear to hold in open contempt. This is not surprising as they are former pirates themselves. The Shichibukai generally do not care about matter's that do not affect them. As part of their duties they are invited to attend meetings with high ranking officers of the Marines. However, as pirates they rarely bother to show up for them.

In exchange for these services, the Shichibukai are pardoned for their former actions and any existing bounties are rescinded.

Manga/anime influences
The Shichibukai are loosely based on the privateers of old Europe. The privateers were approved pirates, considered heroes in their homeland and pillagers in others. Their main goal was to plunder the villages and towns of the then mighty country of Spain.

The concept is also based on the movie The Seven Samurai which is one of Eiichiro Oda's favorite movies.