Stren Withel was a thief and second in command to Ymor who first appeared in The Colour of Magic. They plotted to kidnap and rob Twoflower, who was carrying a lot of gold, unaware of its value.
He was a former trainee of the Assassin's Guild who was expelled for 'enjoying himself too much' when inhuming. He once attempted to kill Ymor, and would have succeeded has Ymor's ravens not attacked, costing Stren his left eye.
Stren Became the President of the Thieves' Guild