Yonko (One Piece)

The Yonko are considered to be the world's 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 assassin subsequently replaced him, while another member stopped the aforementioned war.

Yonko

History

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. It is unknown if the Yonko existed at this point of time. 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 from 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 strongest pirate was coming to an end, warning him of the new era to come. With Marshall D. Teach making his advance to the top and he will depose Newgate from his throne, the pair are seen clashing weapons splitting the skies to the horror of both crews and their members.

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 debilitated 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 resolve 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 have now been reduced to three, and the balance of the Three Great Powers has collapsed, 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 "Iron Boy" Scotch, if attacking him would gain Kaido's attention, Scotch replied with the answer: "Yes". Drake changed into his dinosaur form and began attacking the Cyborg. Also, the Gorosei hypothesized that Blackbeard was the pirate closest to obtaining the vacant position in the Yonko due to his possession of two Devil Fruit abilities and having complete knowledge about Newgate's sphere of influence.

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. When the candy factory was wrecked during the insurrection of the New Fishman Pirates, Fishman Island may face Big Mom's wrath since the island does not have enough candy for the tribute. Monkey D. Luffy has pledged that he will make Fishman Island his territory by defeating Big Mom. Teach has also usurped Whitebeard's position as the fourth Yonko, as the Gorosei predicted, having managed to conquer 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 govern. 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 distrust among them, that keeps their power in check. During the Battle of Marineford, the Marines and Shichibukai needed their full strength to fight against just one of the Yonko and his crew.
The Yonko themselves are not united together as a coalition compared to 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 slave trade and pirate attacks on Fishman Island stopped after the island was declared to be under Whitebeard's control.
After the timeskip Trafalgar Law cited that there are only two methods of surviving in the New World, and that is to be protected serving under the umbrella of a Yonko's dominion or to constantly oppose their authority.