During the Second War, Alexandros Mograine came across a dark crystal which severely scarred his hand as soon as his touched it. He kept the crystal a secret up until the Scourge reached Lordaeron when he revealed it to the other leaders of the Silver Hand suggesting that they should search for the opposite of the crystal, a light crystal however they were disgusted at the idea and attempted to destroy the dark crystal with a holy spell. However, the orb absorbed the spell and the light crystal that Mogriane had suggested just before was formed. They decided that the crystal should be used to form a powerful weapon that could defeat the scourge.
Mograine travelled with High Inquisitor Fairbanks to Ironforge to ask the Dwarven king, Magni Bronzebeard, to forge the weapon for them. He accepted and the mighty sword was forged at the Great Anvil in the center of Ironforge.
For the forces fighting against the Scourge, the Ashbringer was one of the most lethal weapons available in their arsenal. It was so powerful that it caught the attention of Kel'Thuzad who made a deal with Balzanarr to eliminate Alexandros Mograine. He corrupted Mograine's son, Renault Mograine, who then led his father and High Inquisitor Fairbanks to Stratholme where armies of Scourge were waiting for them, Renault fled leaving Mograine and Fairbanks fighting for themselves. They killed thousands of Scourge before they were worn out in fatigue which is when Renault returned and finished off his father using the Ashbringer its self which led the blame to become corrupted due to the betrayal.
Kel'thuzad resurrected Alexandros Mograine as a Death Knight inside Naxxramas, where he served as one of the Four Horsemen, and he reclaimed his blade which was left at the corpse of Alexandros.
Darion Mograine, the second son of Alexandros Mograine, seeked to free his father from the grasp of Kel'Thuzad so he led a group of Argent Dawn members into Naxxramas where he found his father who was hostile towards Darion. He had no choice but to slay his father which resulted in him becoming the wielder of the Ashbringer. Once in possession of the weapon Darion began to hear his father's voice through the sword which led him to his brother Renault Mograine who was located in the Scarlet Monastary as the Scarlet Commander.
Renault attacked Darion and just before he was able to deal the final blow the Corrupted Ashbringer acted on his own, letting out the spirit of Alexandros Mograine in the form of green flames. The spirit beheaded Renault as he begged for forgiveness. Some time later, Darion used the blade to take his own life at Light Hope's Chapel saying the words, "I love you, Dad". He was resurrected in the form of a Death Knight for his noble actions.
A long time after these events, Darion ordered his Death Knights to attack the Argent Dawn under the Lich King's commands. After Darion's forces surrended at the battle to Tirion Fordring, the Lich King appeared threatening to kill Fordring however Darion betrayed the Lich King by giving the Ashbringer to Fordring which overpowered the Lich King forcing him to teleport away. The Ashbringer was now in Tirion Fordring's hands and the moment began the alliance between Darion and Fordring.
Corrupter Ashbringer in World of Warcraft
The Corrupter Ashbringer was a level 60 lootable two-handed sword in Naxxramas prior to patch 3.0.2 which saw the refurbishing of the old level 60 instance into a level 80 instance. It dropped off the Four Horseman, with one member being Alexandros Mograine. However Alexandros was replaced by Baron Rivendare in patch 3.0.2 and the Corrupted Ashbringer no longer dropped in the instance.
Equipping the sword would temporarily give the player a Hated reputation with the Argent Dawn making all of the faction's members hostile towards you and also make the Scarlet Crusade friendly. Furthermore, equipping the weapon inside the cathedral wing of the Scarlet Monastary would result in a special scripted event which tells the story of Renault Mongraine's betrayal of his father.