Location: Reardon Manor, near Lindenvale
Assuming you didn’t kill Letho in your Witcher 2 game save, the man-mountain witcher known as Letho is initially hiding out in a barn on the grounds of Reardon Manor, close to the village of Lindenvale. He moves to Kaer Morhen providing you complete the following:
Secondary Quest: The Fall of the House of Reardon, and Letho lives.