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.
The information you report is dire indeed if the orders you saw are legitimate. We can only hope that his mission was stopped early enough to prevent the enemy from knowing too much about our strengths.
Thank you, <name>. May you remain safe in these dark times.
Mirkblood can be found up the stairs from the repair guy in the Servant's Quarters. For tactics see Tenris Mirkblood.
Note that you must look at the Orders from the Lich King to complete the quest, not simply kill Tenris.