Four Emperors

The Yonko are considered to be the worlds' most notorious pirate captains (by the World Government), but are neither allies nor enemies of one another, preferring to remain independent under most circumstances. These four individually reside within the second half of the Grand Line (known as the "New World"), exerting impressive influence/control over dozens of other pirate crews and self-governing islands. In the climactic Battle of Marineford, one of the Yonko met his end towards the climax of the Whitebeard War, his killer subsequently replaced him as a Yonko and another Yonko stopped the war.

Golden Age of Pirates
The first of the Yonko to be introduced was Shanks who visited Luffy's hometown ten years before the story began. Although, it is unknown if the Yonko 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 the only two men to tie in a fight with Gol D. Roger, the Pirate King. Whitebeard was also later announced as the strongest man in the world upon Roger's death. Some of the Shichibukai fought the Yonko in the past; Shanks and Dracule Mihawk frequently dueled in their younger days, Gekko Moriah fought Kaido, and Crocodile fought Whitebeard.

All of the Yonko seem to have strong fleets, including huge ships and elite crewmen. All Yonko 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 Yonko in the Grand Line.

Until recently, the World Government has been fine with the world powers remaining as they are. With the Yonko 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 Yonko, 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. Kaido attempted to attack 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 Yonko. Shortly afterwards, Shanks arrived at Marineford with his crew and stopped the war.

According to Eustass Kid, due to Whitebeard's death, the Yonko 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.

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

New World Saga
After the death of Whitebeard, Big Mom took over as protector of Fishman Island in exchange for Fishman Island paying a large amount of candy to the Yonko every month. However, due to the coup of the New Fishman Pirates, Fishman Island is under the threat of facing Big Mom's wrath since the island doesn't have enough candy for the payment. Monkey D. Luffy protects the island by declaring war on Big Mom with the intention of making Fishman Island his territory. Teach has also succeeded Whitebeard as the fourth Yonko, having managed to take most of Whitebeard's former territory.

Abilities and Powers
As an entity, the Yonko 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. Individually, they are considered to be the most powerful pirates in the world, as well as having a powerful crew. Because of their power, they can declare chosen locations as their own; 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. These territories benefit from the protection the emperors provide, keeping the borders safe from any faction that might want to invade them. It is only with the combined efforts of the Marines and the Shichibukai, as well as the disunity among them, that keeps their power in check. During the Whitebeard War saga, the Marines and Shichibukai almost needed their full strength to fight against just one of the Yonko and his crew.

The Yonko 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 Yonko forming an alliance is enough of a threat to cause the World Government to panic, as seen when the World Government attempted unsuccessfully to prevent a meeting between Shanks and Whitebeard at one point in the series. The World Government feared that the two Yonko may unite together and create a stronger threat. A war between two of the Yonko is considered an epic crisis, as 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, such as 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, two of the Yonko have shown mastery over Haki (including Haoshoku Haki). It is unknown if the other three Yonko are skilled with Haki as well.
 * Shanks, Whitebeard and Blackbeard are all familiar with the old days of piracy, before Roger's death. It is unknown if the other two Yonko are also from the previous era.
 * Despite being the third one named, Kaido is the only one who has yet to make an actual appearance in the story.

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