Letter Sealed by Sylvanas is a quest item provided for the following quests:
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.
<This letter bears the additional seal of Lady Sylvanas Windrunner>