The Shichibukai were introduced quite early into the One Piece series as seven powerful pirates who roam the seas freely. Unlike regular pirates, they weren't chased by the Navy because these pirates worked for the government and helped them eliminate other pirates. In return, they were allowed to continue living their lives on the seas with great freedom and continue being merciless pirates. Becoming a Shichibukai certainly isn't easy as it requires one to have tremendous strength and an aura that would intimidate other powerful pirates.