This quest is no longer available within the game.
The Argent Dawn Emissary wants you to search the chamber of Tenris Mirkblood within the Servant's Quarters of Karazhan.
Excellent, an experienced adventurer. Precisely what we need.
There has been talk from the Kirin Tor guarding Medivh's tower that something is amiss with Karazhan. There is a new power lurking within, one that answers neither to the Guardian's magic nor the Legion. From what they've scouted, it appears it is an elven prince called Tenris Mirkblood, and there is reason to believe he serves the Scourge.
Go, my friend, investigate Mirkblood's chamber, and return with any information you might find.