From wow.incgamers.com

Quest

Faint Necrotic Crystal: On the corpse of the fallen wight, you find a crystal faintly radiating necrotic energy.

Quest Objectives

Bring the Faint Necrotic Crystal to Lieutenant Rukag in the courtyard of the Undercity.

Conclusion

Lieutenant Rukag: 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.

Rewards

You will be able to choose one of these rewards:

• Blessed Wizard Oil
• Consecrated Sharpening Stone

Location

Lieutenant Rukag can be found in the Ruins of Lordaeron courtyard, by the bridge.