According to semi-canon sources, Lord Denys asked King Aerys II for certain rights for his citizens and a new town charter for Duskendale. He refused and Lord Denys refused to pay taxes to the crown. This led to an escalation of events that saw the capture of King Aerys and the death of Ser Gwayne Gaunt of the Kingsguard. Lord Denys continued his defiance, even as the might of the Seven Kingdoms sat outside his walls.
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.