Do not lose this letter. Reports indicate that this only drops the
first time you defeat VanCleef. People who have killed VanCleef again,
whether or not they had dropped the quest, have reported not being able
to get the letter a second time.
Searching Edwin VanCleef's person, you discover, among other things, an unsent letter. It is addressed to Baros Alexston, the City Architect of Stormwind, City Hall, Cathedral Square.
It appears to be recently written and sealed.
Progress
Yes? Is there something I can help you with?
Completion
Edwin VanCleef, you say? I would as much expect a letter from my dead grandmother... So you've killed him then? Pardon me in saying, but I'm somewhat surprised. He was a peerless fighter in his younger years. Let's see what he finds the need to write to me about after so many years.
<Baros peruses the letter.>
Edwin... I see the years haven't changed you a bit, an idealist as ever, and a romantic. He doesn't care who he hurts, <name>. Revenge has consumed him. But then, I'm not sure I can blame him.
Completing this quest will reward the Deadmines achievement (if for some reason actually killing Edwin VanCleef did not reward it), presumably because it is only possible to obtain this quest by defeating VanCleef.