Letter from Lor'themar Theron is a quest item for  Envoy to the Horde and its non-blood elf equivalent  Envoy to the Horde.
Esteemed leaders of the Horde,
It is with great pleasure that I - Lor'themar Theron, Regent Lord of Quel'Thalas - announce the fall of the Scourge commander known to us as Dar'Khan Drathir.
Dar'Khan's foothold in our lands was brought up by critics of the sin'dorei race at numerous previous negotiations. We hope this event addresses any and all previous objections your graces might have harbored.
PS - Our grand magister has received word back from Outland regarding Thrall's question. The answer is, with great certainty, yes.