Guild Master Williams can be found on the top floor of the Broken Valley Mine Tower. He provides you with a key to the Broken Valley Mine (needed for the main quest, Lock and Key) and tells you about an entrance from the southeast.
Mindread him to reveal: "Am I lucky they never asked for the number combination of the treasure chest in the mine office! I'd have cracked immediately. Maybe I can still save what's left when these monsters are gone!"
At the end of your conversation you automatically begin the quest A Guild Without a Master to help Williams and associates escape the tower, protecting him from the Black Ring members who arrive right on cue.