In One Piece, the sea and therefore the world is governed by the balance between 3 superpowers. These superpowers are: the World Government, the Yonkou AKA Four Emperors, and the Shichibukai AKA what were talking about.
Shichibukai are former pirates that were seen as powerful being by the World Government and is given pardon and military status in exchange for working for them. Some are regarded as heroes, and some are regarded still as pirates. Since the Shichibukai has high military status, they have access to many of World Government facilities like Impel Down.
Because the status of the Shichibukai are based on merit and fame, their roster changes rather frequently throughout the story, mostly due to direct actions from the Straw Hat Pirates.
The first Shichibukai the Straw Hats take down is Sir Crocodile of the Baroque Works. After his loss, he loses his status as Shichibukai and is sent to Impel Down. Since then, and until the end of War of the Greatests, the roster is as follows:
Starting from left: Jimbei of former Sun Pirates, Gekko Moria of the Thriller Bark, Boa Hancock of Kuja Pirates, Donquixote Doflamingo of Dressrosa, Marshall D. Teach of the Blackbeard Pirates, the “Tyrant” Bartholomew Kuma, and the World’s Greatest Swordsman Dracule Mihawk. After the 2 year time skip, the roster changed once more.
This time around, there are 4 major changes. Gecko Moria and Marshall D Teach are no longer present, and Buggy the Clown and Trafalgar Law, a super rookie, has joined the ranks. And there is no seventh warlord….
I hope you enjoyed this little short!