It has already begun. Levixus stands atop the ruins of Auchindoun and reads from the book of the dead. He calls the dead out by name and, through demonic ritual, binds the tortured souls to his will. He will raise all of Auchindoun if given the chance!
But... you must first prepare. You cannot do battle with that which you cannot see.
<Nitrin hands you a parchment.>
Bring me the items I require to brew the tincture. It will open your mind and allow you to see into the spirit world.
Reward
Levixus's army grows in size and power with each second that passes. You must hurry.
Completion
Stranger... I was not forthright with you. Once you drink this potion, you will forever see into the world of the dead. This recipe that you drink of is a rite of initiation for all death priests of Auchindoun. Many go mad after its effects have taken hold. Seeing the dead is ... shocking.
You will also get the ability to see a dead vendor in Auchindoun, Horvon the Armorer, where you can get repairs.
Notes
The suggested players is 3, but you can easily do this on your own. The only reason it is suggested as 3 is because of one Elite Mountain Gronn you have to kill, but he really isn't that tough! All of the items can be found (dropped) near each other in West Nagrand near Warmaul Hill.
The Mountain Gronn can be found on the path leading up to Warmaul Hill. The Aged Clefthooves and Greater Windrocks can be found just at the base of the same path. Aged Clefthooves can also be found around the back of Oshu'Gun, on the route where Durn patrols.
Just keep killing them and all items will be dropped eventually.