Bring an Azerothian Diamond and a Pristine Black Diamond to Lord Grayson Shadowbreaker in the Cathedral District of Stormwind.
We now must assemble a Divination Scryer - this will begin the trial that will allow you to redeem the fallen charger. Your scryer, actually, is almost done. The exorcism censer used in Terrordale is the catalyst for it, and I will fashion a suitable housing for the device.
The last components needed, though, are two diamonds. They must be as juxtaposed in brilliance as they are in purpose - an Azerothian and a Pristine Black. These two will be the beacon by which the scryer will pierce the shadows!
You have come so very far, <name>. Your final act of worldly-item sacrifice will be the genesis for one of your greatest accomplishments. I can feel it in the very fiber of my being!
These are fine specimens, <name> - the final pieces are now in place. At long last, your time has come!
In the depths of the Great Ossuary of Scholomance are the skeletal remains of many beings. Amongst them is a once-noble charger. The hollowed soul of this charger now serves as the steed for Death Knight Darkreaver. It is he whom you will face, and it is this steed that you will pass judgment upon. Only you, <name>, can redeem its soul and save it from the torment of its servitude.