WoWDB

Very easy quest.  Just be mindful of the range between your character and the sentries while making good use of  Sap.

Video guide for those don't want to read:

You will need to travel to the right around the tower and use the rope in the grass to scale up to the roof.  (The game takes control of your movement and your camera while using the rope.)  From there, click on the unprotected vent pretty much in front of you.  Re-enter Stealth and travel down the spiral stairs.  Jump off at the first landing and sap a patrolling Ravenholdt NPC if necessary.  The stairs into the basement should be to your right and underneath the landing you just jumped from.  A cutscene (see video) plays as you descend those stairs.  (You can skip past it if you so choose by pressing Esc.)  Turn in the quest to Wrathion once he walks up stairs to where Fahrad is standing.

Travel back into the cave path that leads to Ravenholdt Manor and speak to Mostrasz if you need to reacquire the quest.