*spoiler alert* So i killed the huge rat in the sewer and i came to the part where i face the second brother in the hamlet and the trickster guy. I killed them both and all the other rats
that had spawned but the letter didnt drop. I am almost positive that the first run i did the second brother drops a letter that says i am the one that killed his bro. On the ground where i killed him only his huge cannon dropped. And the trickster didnt drop anything. Just to make sure i killed all the regular rats too but no letter anywhere