The Defiance of Duskendale occurred in 276 AL when Lord Denys Darklyn of Duskendale demanded a new town charter from King Aerys II Targaryen, refusing to pay any more taxes.
When Aerys came to Duskendale to arrest and execute Lord Denys, the latter actually kidnapped and imprisoned the king, which some say was what drove Aerys mad. He was saved by Ser Barristan Selmy, after which all of House Darklyn and House Hollard were destroyed.