This quest is not available in game.
Kill Fenwick Thatros and bring his head back to Mennet Carkad in the Rogues' Quarter of the Undercity.
Lord Varimathras has just sent me a missive about a threat to the Forsaken that must be immediately removed. His name is Fenwick Thatros, one whom Lord Varimathras believes is a leader of the undead that have taken root in Silverpine Forest.
Your name came to mind immediately, <name>. I have faith enough in your abilities that you will carry out this mission flawlessly.
The latest reports from the Deathstalkers in Silverpine indicate that Thatros can be found near the decrepit dock on Lordamere.