Find the traitors and recover their artifacts, then return to Magus Voidglare in Tarren Mill.
I came to Tarren Mill to research, but now must resolve a crisis. You see, four Forsaken fled the Undercity a few months ago. They turned their backs on their brethren, but what's worse...they stole from the Dark Lady.
These thieves broke into a secure vault and stole four artifacts, items our apothecaries required in certain studies. Sylvanas would have these artifacts returned.
The thieves fled the Undercity to Dalaran, and those wizards quarantined them in the Lordamere Internment Camp.