The Thief-Lord was a powerful man among the gangs on the streets of Deadman's District in Hajra. He commanded a gang called the Lightnings. After the death of his mother the Thief-Lord took Briar Moss, who had been known as Roach in that time, in and taught him the ways of a thief. The Thief-Lord also gave Briar his street name, Roach.
The Thief-Lord also worked with the best spells money could buy to pass invisible among his subjects to listen to their conversations, learning about their secrets and if any of them plotted against him. He also ruled his subjects with a firm hand, punishing them for mistakes and even "tearing someone to pieces" when a boy broke into his treasury.
He doesn't appear in any of the novels but has only been mentioned various times by Briar, when he was talking about his childhood in Sotat.