The Shepherd

"There the demons dwell, this is their city. If you would make it yours, destroy them."

- The Shepherd to the rioters

Biography
During the Dance of the Dragons, King's Landing's smallfolk began a riot against Queen Rhaenyra Targaryen, demanding vengeance for the deaths of Helaena Targaryen and Jaehaerys Targaryen. In the midst of this chaos, The Shepherd began to rant against Rhaenyra's dragons, and by extension, all dragons. He pointed to the top of Rhaenys's Hill, where the Dragonpit stood. He declared that King's Landing was the dragon's city, and if it would belong the people, they must kill the dragons. Ten thousand people cried out, "Kill them!".

The ensuing chaos led to Storming of the Dragonpit, which led to the deaths of Prince Joffrey Velaryon and five dragons.

In the books
It was the fear of dragons, and of their presence, that gave birth to the Shepherd. His name has been lost to history. Some suppose he was a poor beggar, others that he might have been one of the Poor Fellows who, though outlawed and disbanded, still stubbornly haunted the realm. Whoever he was, he began to preach in the Cobbler's Square, saying that the dragons were demons, the spawn of godless Valyria, and the doom of men. Scores listened—then hundreds, then thousands. Fear begat anger, and anger begat a thirst for blood. And when the Shepherd announced that the city would be saved only when the city was cleansed of dragons, people listened.

The Dance of the Dragons
During the riots in King's Landing the Shepherd began to rant against dragons, not just the dragons that were coming to attack the city but all dragons everywhere. The half-crazed crowd listened to him preach and after he was done ten thousand throats cried "Kill them! Kill them!", and much of the mob headed for the Dragonpit, where three of theblacks' dragons and one of the greens' dragons dwelled.

After the storming of the Dragonpit and Rhaenyra's flight, the Shepherd and his mob ruled much of the city.