The Defiance of Duskendale was a conflict during the reign of King Aerys II Targaryen. During it, Ser Gwayne Gaunt of the Kingsguard was killed by Ser Symon Hollard, and avenged by Ser Barristan Selmy.
In the books
The Defiance of Duskendale was a landmark event during the reign of Aerys II. It occurred somewhere around the year 276 AC. It began when the Lord Denys Darklyn of Duskendale, seemingly at the behest of his wife, Lady Serala, refused to pay taxes, demanding a new town charter and certain rights for citizens and ended with the extirpation of two noble families - House Darklyn and their vassals, House Hollard - and a severe rift between the King and his Hand of the King, Lord Tywin Lannister.
Since Aerys wished to distance himself from his Hand, Lord Tywin, he decided to deal with the Darklyns himself. Aerys went to Duskendale with the Kingsguard and a small force of men, to arrest and put to death Lord Denys. Instead, Aerys was imprisoned. During the capture, Ser Symon Hollard slew Ser Gwayne Gaunt of the Kingsguard. Lord Denys continued his defiance, even as a large host sat outside his walls, commanded by Tywin Lannister. Tywin's ability to act had been paralyzed when Lord Denys sent word that at the first sign that Tywin intended to storm the town, Lord Denys would kill the King.
The Defiance ended when Ser Barristan Selmy sneaked into the Dun Fort, the seat of House Darklyn, and rescued his King. Lord Denys, no longer having a hostage, immediately opened his gates and begged for mercy. He was beheaded, along with the rest of his family. Not one Darklyn remained alive. The same for House Hollard, who had sided with House Darklyn during the defiance, with the exception of Dontos Hollard who was a child at the time. Ser Barristan asked for his life to be spared and as he had saved King Aerys, he could not refuse him.
Duskendale was then granted to House Rykker.