From Overview:
The PCs have returned to the Nezumi network where the war has come to a stalemate. One night when they drink at the "Eternal Dragon Inn", a new establishment on the surface that caters to humans, they come across an old enemy and fighting breaks out. During the tumult, the son of a powerful mercenary leader is killed by an unknown assassin and the PCs are blamed for his death. Only if they find the true murder the mercenary captain will let them go.
You can use different motives for the murder, but when used within the context of this campaign, the reason is that the forces of the dark spirit want to bring disorder into the region which would open a direct path of attack from the lands of the Red Daimyo onto the Northern Empire.