How is Lord Darcy connected to Master Sean O Lochlainn in the novel, Too Many Magicians?

Lord Darcy is an investigator for the King's Justice, whereas, Master Sean O Lochlainn forensic sorcerer. While Darcy has the superior gifts of deduction, and knowledge of self-defense, interrogation, and tactics, he must rely on Master Sean's scientific expertise and psychic gifts to gather the necessary evidence upon which to base his deductions.