Four Emperors

The Yonkou are the four (currently three) great pirates who rule the second half of the Grand Line (known as the "New World").

History
The first of the Yonkou to be introduced was Shanks who visited Luffy's hometown ten years ago before the story. Although, it was unknown that the Yonkou existed at this point of time. It is reasonable to assume they did as this was around the time the Shichibukai organization was being formed. The second was Whitebeard, one of only two men to tie in a fight with Gol D. Roger, the Pirate King, the other being Garp. Whitebeard was also later announced as the strongest pirate in the world upon Roger's death. Some of the Shichibukai fought the Yonkou in the past; Shanks and Dracule Mihawk frequently duelled in their younger days, Gekko Moriah is rumored to have fought Kaido, and Crocodile fought Whitebeard.

All of the Yonkou seem to have strong fleets, including huge ships and elite crewmen. All Yonkou have traveled to the New World at some point in their past, where they are currently locked in stalemate, unable to progress any further into the Grand Line. This is a product of the combined efforts of both the Marines and Shichibukai, as well as the presence of the other Yonkou in the Grand Line.

Until recently, the World Government has been fine with the world powers remaining as they are. With the Yonkou stuck in a deadlock situation, this allowed the world to remain stable. However, with the absence of Crocodile, defeated at the hands of the Straw Hat Pirates, the World Government was alarmed that two of the Yonkou, Shanks and Whitebeard, arranged to meet each other. Later, with Enies Lobby in ruins, the Marines saw another defeat in the New World when a blockade failed to prevent the two meeting.

Fearing the two crews might form an alliance, the World Government lay in wait to see what happened. Shanks believes the days of Whitebeard as the top pirate was coming to an end, warning him of the era to come. With Marshall D. Teach making his move to the top, the pair are seen clashing weapons.

Whitebeard War Saga
In an attempt to rescue his 2nd Division Commander, Portgas D. Ace, Whitebeard attacked Marineford. Kaidou attempted to stop Whitebeard; however, he was intercepted by Shanks. During this battle against the Marines, Whitebeard revealed how ill he had become over the last two decades, and that he continues to suffer more critical damage from his adversaries in his weakened state. Remaining at Marineford alone to finish the conflict with the Marines, Whitebeard was ambushed and ultimately perished at the hands of the Blackbeard Pirates, leaving only three Yonkou. Shortly afterwards, Shanks arrived at Marineford with his crew and stopped the war.

According to Eustass Kid, due to Whitebeard's death, the Yonkou has now dropped down to three, and the balance of the Three Great Powers have crumbled, which led to chaos around the world. Indeed, islands that were once under Whitebeard's protection, such as Fishman Island and Foodvalten, were under attack by pirates.

A little while after the war ended X. Drake arrived at an island belonging to Kaido, and is one of his favorites. Upon asking a cyborg there, if attacking him would gain Kaidou's attention. The cyborg replied with the answer: "Yes". Drake changed into his dinosaur form and began attacking. Also, the Gorousei believed that Blackbeard is the one closest to filling in the empty spot in the Yonkou due to him possessing two Devil Fruits and having complete knowledge about the New World.

New World Saga
After the death of Whitebeard it is revealed that Big Mam took over as protector of Fishman Island in exchange for Fishman Island paying a large amount of candy to the Yonkou every month.

Strength
As an entity, they are one of the three great powers of the world, the others being the Shichibukai and Marines. Their strength and influence are such that they live quite literally as though they are emperors, doing mostly as they please. They are considered to be the most powerful pirates in the world, individually as well as a crew. Because of their power, they can declare chosen locations as their own; and the mere declaration is often enough for others, both Marines and Pirates alike, to acknowledge their authority. In fact, in the second half of the Grand Line ("New World"), they each have a territory that they rule. It is only with the combined efforts of the Marines and the Shichibukai, as well as the disunity amongst them, that keeps their power in check. During the Whitebeard War saga, the Marines needed their full strength to fight against just one of the Yonkou.

The Yonkou themselves are not united together as one group like the Shichibukai and follow their own individual paths. The possibility of two or more of the Yonkou forming an alliance is enough of a threat to cause to the World Government to panic. For that reason, the World Government attempts unsuccessfully to prevent a meeting between Red-Haired Shanks and Whitebeard at one point in the series. The World Government feared that the two Yonkou may unite together and create a stronger threat. A war between two of the Yonkou is considered an epic crisis. Shanks' interception of Kaido's advancement on Whitebeard had taken the Marines by shock and awe; putting the World Government in a 'cold sweat' as stated by one of the Marines.

They can inspire others to become pirates. Examples of this include Shanks inspiring Luffy. On top of this, the influence these pirates have over others is far greater than any other noted group. With one declaration, the kidnappings on Fishman Island stopped after the island was declared to be under Whitebeard's control.

Trivia

 * So far, none of the Yonkou have had their bounties revealed.
 * So far, two of the Yonkou have shown mastery over Haki (including Haoushoku Haki). It is unknown if the other two Yonkou are skilled with Haki as well.

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Yonkōu Yonkou 四皇 Quatre Empereurs Imperatori