A couple months after the Princess's visit, a chain of events that would eventually lead to the fall of the throne started. The Defiance of Duskendale they called it. Lord Denys Darklyn ceased to pay taxes to the throne, and he knew that King Aerys and the hand of the king, Lord Tywin Lannister, have had a strained relationship for the past few years.
So he came with a devious plan: he had invited the king to Duskendale to hear his petition. Lord Tywin Lannister obviously warned him not to go, but the king didn't listen, as the king grew jealous of Lord Tywin's accomplishments. So he decided to make a name for himself in solving problems, so he journeyed to Duskendale, along with a small band of men to guard him.
But when he arrived, the king was imprisoned by Lord Darklyn. His guards were slaughtered, and the king was stripped of his royal garments, shoved roughly, even a squire named Robin Hollard pulled out his beard.
Lord Darklyn refused to give up when a messenger arrived, he threatened Lord Tywin that if there was any sign of assault, he would kill the king immediately. So Lord Tywin besieged Duskendale instead, as he could not attack it.
The defiance lasted for six months, Lord Darklyn was convinced that Lord Tywin could not do anything to him, as he had the king, but Tywin gave Duskendale one final demand for surrender, and if he didn't surrender, the loyalists would storm the place, and kill everyone inside it. Lord Darklyn refused.
Eventually, Ser Barristan Selmy offered to go on a solo rescue mission, Lord Tywin gave him a day before he would storm the castle. And at night, Ser Barristan climbed the walls discreetly, making his way to the Dun Fort disguised as a hooded beggar. He freed the king out of the dungeons of the castle, and quickly got out of Duskendale. Without a hostage, Lord Darklyn surrendered. His punishment was terrible. King Aerys almost wiped out their house and their kin, the only one who survived was Dontos Hollard.
King Aerys was a good king, but when the Defiance of Duskendale happened, he changed, becoming more erratic and paranoid as time went on. His judgement became more severe and cruel, and from 277 AC, the king never left the Red Keep until the Tourney at Harrenhal in 281 AC.