Quel'Delar, The Sister Blade
Quel'serrar was gifted to the night elves, but nearly destroyed. It remained partially intact for thousands of years with the Shen'dralar of Eldre'thalas. The Shen'dralar hoped that the blade would someday be restored. It wasn't until recently that heroes reforged Quel'Serrar in the flames and blackened heart of Onyxia, Broodmother of the Black Dragonflight. The restoration of the sword brought joy to the night elves who feared it had been lost forever.
Quel'Delar, on the other hand, had a much different story...
Quel'Delar was reverently carried by the night elves, the same elves that eventually became the first High Elves. The blade was passed down from father to son as an heirloom amongst their people until it came into the hands of who would be the last warrior to hold the weapon, Thalorien Dawnseeker. He valiantly used the weapon to defend Silvermoon against any who dared try to attack his beloved city. He was able to defend his city until the Third War. It was during the Third War that Arthas and his Scourge forces marched on Silvermoon. Thalorien fought bravely and valiantly but the scourge forces proved to be too much for him. Eventually he was overcome and slain. Quel'Delar was lost in the aftermath.
The High Elves eventually overcame the devastation and started reconstruction on Silvermoon under the reign of Kael'Thas Sunstrider. During this period of rebuilding, they renamed themselves Blood Elves, in honor of their fallen brothers and sisters. It was in the midst of reconstruction that a veteran warrior named Lana'thel found Quel'Delar burried in the rubble. Now back in the hands of an elven warrior, Quel'Delar was returning to its former glory.

To avenged the death of her people, Lana'Thel accompanied Kael'thas as he made his way to Northrend to defeat the Lich King. Things didn't exactly work out in their favor, however, and Arthas defeated both Illidan and Kael'Thas, leaving Lana'Thel to face the Lich King alone. She fought honorably, but was no match for Arthas and was slain. He then raised Lana'Thel as Blood Queen Lana'Thel with Frostmourne's powers, making her the first of his San'layn Darkfallen.
Though she was almost completely under the Lich King's control, Lana'Thel was reminded of her life in Silvermoon each time she saw Quel'Delar. In a moment of insurmountable grief, she shattered the blade into pieces, spreading them across the frozen lands. These pieces of the sword were never to be recovered.
Amongst the armaments of the Icecrown Citadel, the hilt remained and it was eventually discovered by heroes who were attempting to fight their way through the stronghold to face and hopefully defeat the Lich King. The heroes took the hilt to the elves of Northrend in hopes of discovering the origin of the spectacular hilt. The Sunreavers and Silver Covenant helped to reveal the origins of the blade through recovered ancient texts.
With the history of the blade now revealed, Krasus of the Wyrmrest Temple confirmed that the hilt was indeed the hilt of The Sister Blade, Quel'Delar. Kalecgos argued that this blade was too powerful to be left in the hands of mere mortals, but Alexstrasza convince him, eventually, that there were no better hands than those of mortals for the blade to be in. It was then decided that the blade was to be restored through whatever means necessary. Upon hearing this news, Blood Queen Lana'Thel disgraced the blade at Quel'Delar Rest and vowed that i twould never serve another.
The elves would not give up on restoring Quel'Delar and reforged the blade with infused Saronite from Arthas' Pit of Saron and after defeating Forgemaster Garfrost, used his hammer to shape the blade. The blade was tempered in the Crucible of Souls but this did not bring it's ancient powers back. The blade did, however, show sparks of life, leading the heroes into Arthas' private chamber, The Halls of Reflection. Upon entering these frozen halls, Quel'Delar began feeding off of Frostmourne's evil energy.
Uther the Lightbringer's soul appeared before the heroes, scolding them for their foolishness. He stated that forging the blade of saronite, an evil ore - blood of Yogg-Saron, was a dangerous thing to have done. He explained that now that Quel'Delar had been exposed to Frostmourne's evil, the only way to purify the blade was to take it to the Sunwell within Silvermoon City. The heroes did exactly this.
They were initially denied entrance to the Sunwell by Halduron Brightwing. He eventually realized the importance of restoring Quel'Delar and asked the heroes to seek the blessing of the last true owner of Quel'Delar, Thalorien Dawnseeker. Upon visiting his remains, the heroes were thrust into a vision of Thalorien's last stand against the scourge invasion and they fought at his side against the waves of scourge. With his spirit finally at rest, Thalorien gave his blessing to Quel'Delar, allowing it into the Sunwell and to gain a new owner.
As the heroes purified the blade in the light of the Sunwell, Regent-Lord Lor'Themar and Grand Magister Rommath watched, awe-stricken. The blade was finally restored to its former glory and ancient power. Theron tried to wield the sword, but it struck him and sent him flying across the room. The captain of the guard explained that Quel'Delar chooses it's own master and it had chosen the hero that reclaimed it.
At long last, Quel'Delar was resurrected and ready to avenge Quel'Thalas one last time!
(Adapted from "Know Your Lore" at wow.com)