It seems that your Assassin X failed to finish the deed. If Messere insisted to know the incompetent of your assassin, then I am gladly to write it out for you.
During my investigation of the disappearance of Kirkwall's Champion, one would too fear to go out from their house after the battle of Kirkwall incident. Mages rise up and templars are everywhere. But your assassin is fool enough to venture out the city, seeking information from the villagers and the nobles. His battle against the guards and templars were rather satisfying and need trainings, a training that I fear for your assassin wouldn't need any longer.
As I follow the assassin trails, I never get spotted nor I doubt that's possible. The assassin did suspected but I made sure I hid myself well. As soon the assassin reached the targeted estate, the assassin let his guard down and collapsed to the solid ground between my feet. Upon looting the dead body, I found the location to the assassin's hideout.
Do not worry, he was clarified as a victim under the templars, I made sure of that. You scheme is still safe with me, the letters I looted indicates several evil plots of yours against the noble you are planning to assassinate. I will not take kindly to those who threatens my friends, you have made a grave mistake.
I do not possess weakness for greed, and I will not take lightly against threats. Enjoy your remaining days to come as I will venture to seek you out, myself.
The Eagle's Eyes.