Ymor appeared in The Colour of Magic as the head of the Thieves' Guild at the time. He used a network of ravens as messengers and guards, and the ravens are notoriously loyal to him. His second-in-command, Stren Withel, was ambitious, and nearly killed Ymor at one point, if not for the assistance of Ymor's ravens, which caused Stren to lose his left eye. He also had a demarcation agreement with Zlorf Flannelfoot, then head of the Assassins' Guild: the Assassins wouldn't rob, and the Thieves wouldn't kill people, at least for money.
When Twoflower arrived in Ankh-Morpork, news of him quickly came to Ymor through Cripple Wa, a beggar. After receiving another report from Gorrin the Cat, a thief stationed on top of the Temple of Small Gods, he decided to pay a visit to the Broken Drum later in the evening to rob Twoflower- who had around 2,000 rhinu. At first, things went well, but once Zlorf Flannelfoot and several other Assassins turned up, tensions began to rise. After the arrival of Rerpf and some other representatives from the newly formed Merchants' Guild, all three groups began fighting each other.
At that point, the Broken Drum caught fire, and Twoflower and Rincewind fled the city, and Ymor lost all hope of getting Twoflower's gold...