The content of the letter was as follows:
"Dear Mr. Ethan, I am glad to inform you that I have discovered your secret of selling all those sentient swords and buying mana orbs in large quantities. It was very astonishing to me that you're the owner of those D-Grade sentient swords because I learned that you're just an E-Rank hunter.
So, if you don't want your secret to be leaked, meet me at the Eldritch Chamber of Commerce.
But, You don't need to fear Mr. Ethan. Instead, I have a deal that you would very much like to hear.
I hope you will meet me soon enough.
From— your well wisher."
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Ethan was tense at first, but at the later part he couldn't help but find it amusing. It was a really polite threat. Feeling intrigued, he decided to visit the place to see what awaited him.