The Trail of Bones - Chapter 4

The Druchii were seen taking a Nehekharan reliquary this way, but we have besieged them on the hill. Their forces were unable to access the Isle of the Dead via the Urithair Threshold. If what you say is true, then there is a great risk to the vortex, and as such you should do everything in your might to stablize it. This may require some sacrifice at the Isle of the Dead. After you stablize the rituals at the Isle of the Dead, you can seek out the chest - though I suspect it will be empty like the other three you have found before.

Travel to the Isle of the Dead and complete the four rituals there, that is the Rituals of Beasts, Heavens, Death, and Life. Afterwards find the Nehekharan Artifact on the butte north of Caelanriol.