The most critical keepsake Hannibal Lecter ever removed from a victim was Will Graham’s ravenstag.
Hannibal is a show about the pain of empathy, and "Mizumono" hurts. The finale brings the Ravenstag story arc to a conclusion in a brutal way.
When it comes to understanding exactly what to expect from Hannibal, the two most important characters are symbolic representations of who Will Graham and the titular serial killer are not. At least not anymore.