Bring the Faint Necrotic Crystal to Lieutenant Rukag in the courtyard of the Undercity.
On the corpse of the fallen wight, you find a crystal that is faintly radiating necrotic energy.
Can I help you with something?
You say this was taken from a wight attacking the city?
We have received word of the necropolis base camps that are being summoned by the Scourge. I suspect they were trying to create one in the Undercity. We will have to be vigilant in the future.