If you specifically meant Season 7, Episode 3, the "piece" of evidence crucial to that case involves:
Spencer Reid is cast as "Sir Percival" and is given the task of asking the question that will "heal" the King. Context for S7E3: " Dorado Falls " [S7E3] The Fisher King
A man suffering from Capgras Syndrome (believing his loved ones have been replaced by imposters) after a traumatic brain injury. If you specifically meant Season 7, Episode 3,
In the context of the popular crime drama Criminal Minds , the "piece" usually refers to the physical puzzle pieces sent by the unsub Randall Garner—the Fisher King—to the BAU team. The "piece" for the puzzle was a section
The "piece" for the puzzle was a section of a large map or picture that, when assembled, revealed the location of his "Grail"—his daughter, Rebecca.