The Kingslayers, also known as Assassins of Kings, were an organization made up of a handful of witchers from the School of the Viper in the service of Nilfgaardian Imperator Emhyr var Emreis. The organization's task was to spread chaos in the kingdoms of the North by killing kings and sorceresses with Letho of Gulet as an informal leader.
The group was once saved by Geralt, and Geralt learns he turned over Yennefer of Vengerberg to their care. They eventually made their way into Nilfgaard, where they were given a mission by Vattier de Rideaux and the Imperator Emhyr var Emreis, to destabilize the Northern Kingdoms by assassinating monarchs. In exchange, the Emperor promised to rebuild their witcher school. Letho agreed, gathered his comrades and the witchers arrived in a North.
Originally Letho planned Esterad Thyssen, king of Kovir and Poviss, to be the first victim but before he could progress to slay him, he was hired by Síle de Tansarville and Philippa Eilhart to assassinate Aedirnian King Demavend III. At the same time, another kingslayer approached Vizima to assault Foltest, king of Temeria.
Foltest and Demavend
On September 1270, a masked kingslayer sneaked to Royal Palace in Vizima and attempted to kill Foltest but monarch was defended by Geralt. The assassin was very skilled and fought using two swords. The battle was pitched and the outcome did not seem at all certain, but at the last moment, Geralt threw his recently acquired coin purse. This ploy startles the assailant sufficiently to allow Geralt an unparried blow. In the aftermath, both Geralt and King Foltest looked closely at the man's face which has now been unmasked. Inexplicably, the man appears to have the same eye mutation as Geralt.
In spring of 1271, during a bright day, Demavend III was killed on his ship by Letho, who defeated king's knights Himsbach and Anselm, mage Abelard of Hagge and archer Tybalt of Vengerberg. Then he proceeded by cutting his head to use it as proof. It's possible that a crew of nearby ship saw it and would go against the killer but he managed to flee the scene with a help of Iorveth's Scoia'tael commando.
When Letho learned that Foltest was still alive, he devised a plot to kill him, using an identity of a blind monk seeking shelter at solar of La Valette Castle. He was aware of a political situation and knew that, eventually, Temerian monarch would besiege the castle in hope of regaining his children, Boussy and Anais La Valette, back. This happened on April 1271, and when Foltest walked to the solar and turned his back to a monk in pray, he cut him down and even Geralt was not quick enough to help the king.
Attempt to kill Henselt
He then had the other witchers, Serrit and Auckes, attempt to assassinate King Henselt, hoping to pin the further murders on the sorceresses as well. They were preparing for some time but they were ultimately unsuccessful, dying in the process.
As a result of their actions, at least two kings were murdered leaving Aedirn and Temeria in turmoils over a succession. During the Summit at Loc Muinne, mages, Lodge of Sorceresses in particular, were framed as the ones who hired "the mysterious assassins" and soon after witch hunts ensued.
After the deaths of most its members, the organization was dismissed and ceased to exist, with Imperator abandoning his promise of rebuilding the southern witcher school as soon as it was possible.