Quel'Serrar

Quel'Serrar ('high blade' in Thalassian) is an ancient blade, forged by night elves and five dragonflights long ago and gifted to the kaldorei. Its twin, Quel'Delar, was similarly gifted to the quel'dorei. For thousands of years, Quel'Serrar has been kept in the care of the Shen'dralar Ancients, waiting for one capable of restoring the blade. The blade is now rumored to be borne by a new master.

Although dated, it is rather rare to see an "original" Quel'Serrar, and is considered a testament to Classic Raiding.

Source
Quel'Serrar is a quest reward from The Forging of Quel'Serrar (2). The quest is started by using an item that drops from Dire Maul bosses, Foror's Compendium of Dragon Slaying. The droprate was so extremely low that it was considered impossible to farm for it. Since the book was not bind on pickup it could also be found in the Auction House. Only warriors and paladins are eligible for this quest.

The quest starts by handing in the book to Lorekeeper Lydros in the library of Dire Maul, who gives the player the Unfired Ancient Blade. The player must take this blade to Onyxia's Lair, and place it on the ground where it needs to be hit by one of Onyxia's Deep Breath attack. Once hit, the blade turns into a Heated Ancient Blade, which can be picked up and stabbed into Onyxia's corpse once she has been slain to turn it into a Treated Ancient Blade.

Returning this blade to the library in Dire Maul rewards the player with Quel'Serrar.

Trivia

 * In Heroes of the Storm, Sonya, a character from Diablo, wields Quel'Serrar and Ashkandi, Greatsword of the Brotherhood as part of her Wrath Sonya skin.