An empty scabbard has been endowed with various illusory values (invincible secrets, huge treasures) just because it is related to the legend (the relic of the "Scabbardless Swordsman"). This triggered the coveting and pursuit of the four murderers, which eventually led to their deaths.
Concept: The things that people chase crazily, even at the cost of their lives, are often just phantoms projected by their own desires.