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Ashbringer, the Blade of the Scarlet Highlord, is an object of great mystery and power.

One of the previous Highlords of the Scarlet Crusade, Highlord Mograine was known as the Ashbringer. Archmage Timolain crafted him a sword meant to be a bane against all the undead. It (and he) were called such because when he would take to the battlefield, his frenzied blows would fell the Scourge so fast that he would be surrounded in falling ash.

Several characters have mentioned the Ashbringer (or its maker Timolain) on occasion, but the only beings believed to know the current location of the Ashbringer are Nat Pagle, Master Fisherman, and Prince Tortheldrin, the leader of the Shen'dralar within Dire Maul. Getting the full story from them has proven difficult, but Tortheldrin's followers claim that he may be willing to tell what he knows if a player were to defeat the dragon Nefarian and share the tale with him. Some have brought Nefarian's head to Tortheldrin, though he said nothing that lead to the Ashbringer.

It now seems that Highlord Mograine was corrupted by Kel'Thuzad and serves him as a death knight in Naxxramas. He still wields the Ashbringer, though the blade has become corrupted as well.
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The Quest for the Ashbringer

Link to the quest: http://wowwalkthroughs.free.fr/Ashbringer-Quests/Ashbringer-Quests.html

A Korean guild on the public test realms for patch 1.11 supposedly recovered the Corrupted Ashbringer and began the questline for the sword.

Equipping the Corrupted Ashbringer apparently makes the player hostile with the Argent Dawn, but friendly with the Scarlet Crusade, allowing the player to walk into the Scarlet Monastary and talk to Scarlet Commander Mograine, the son of Highlord Mograine.

After a scene involving the spirit of Highlord Mograine and his son, the player can talk to Inquisitor Fairbanks, the undead priest hidden in a room at the back of the Cathedral. He tells the player the story of Ashbringer, how Highlord Mograine was killed, how the Ashbringer was corrupted, and how the player can obtain a new, uncorrupted Ashbringer; seek out Highlord Mograine's other son, who dwells in Outland.

This would mean that Ashbringer will only be properly attainable in The Burning Crusade.

However, the story that Fairbanks tells the player would seem to contradict that which Highlord Mograine himself says, and that which Eligor Dawnbringer says, about Scarlet Commander Mograine being corrupted and driven by revenge. In the case of Dawnbringer, he could simply be unaware of the truth; that Highlord Mograine was in fact murdered by his own son. This could be a translation error, as the only known information on the Ashbringer quest is from a Korean player's screenshots, which have been translated by English-speaking players. Now that patch 1.11 is live, we shall see how the quest unfolds on an English-speaking server.