Ulthalesh, the Deadwind Harvester (quest)

Objectives
Start your search for the Deadwind Harvester at Manor Mistmantle in Duskwood.
 * (Optional) Take the Dalaran portal to Karazhan
 * Investigate Manor Mistmantle in Duskwood
 * Convince Revil to help
 * Duskwood Scroll (provided)

Description
The book says the Dark Riders were last seen in a place called Manor Mistmantle. If friend can find these riders, then  can get Ulthalesh!

Calydus will wait here and await your return. Find Ulthalesh, friend!

Rewards
You will receive:
 * 15,050 XP (at level 100)
 * 15,050 XP (at level 100)

Progress
And just why should I help you, warlock?

Completion
I am left with little choice. I cannot face another Dark Rider ambush alone, and you are obviously formidable.

I will help you find the weapon you see, but only if you will allow me to return the others in their possession to their rightful place.

Do we have a deal, warlock?