Frostmourne

"Frostmourne hungers."

- The Lich King about the Frostmourne sword. "Whomsoever takes up this blade shall wield power eternal. Just as the blade rends flesh, so must power scar the spirit."

- An inscribed warning on the Frostmourne's pedestal.

The Frostmourne is a was a powerful, icy sword crafted by the demonic Nathrezim, long ago. This cursed runeblade, along with the Helm of Dominance and Ner'Zhul's Chest Piece, were encased in a block of ice at the top of a spire, called the Frozen Throne. Ner'Zhul, an ancient Orc Shaman, was killed, and his spirit trapped within the helmet.

When Ner'Zhul learned of the human Paladin, Arthas, he thrust Frostmourne out of the Frozen Throne, and found itself within a cave in the region known as Dragonblight. Arthas, leading an assault on the demon, Mal'Ganis, learned of Frostmourne through the tales he heard from his friend Muradin Bronzebeard. Arthas and Muradin set off to find Frosrmourne, which they believed could help them against Mal'Ganis and his undead minions.

The two of them found the cave that the sword was said to be kept, but was defended by a mysterious elemental creature known as a Revenant. When Arthas and Muradin defeated the Revenant in battle, its last words told Arthas that he was not defending the sword from Arthas... But defending Arthas from the sword! Arthas ignored the warning, and entered the cave. He and Muradin found the sword frozen in a block of ice, as well as a warning. On the wall was a warning that told of a curse that would corrupt one's soul should they wield the sword. Arthas ignored it, and the ice shattered. An ice sharp impaled Muradin, presumably killing him. Arthas grabbed the sword's hilt and returned to the base camp. There, he led the final battle against Mal'Ganis, resulting in the defeat of the demon. Arthas ran off into the icy planes, and returned home months later.

Like the warning said, Arthas had been corrupt by Frostmourne, which he used to murder his own father upon returning. Arthas, now a follower of Ner'Zhul and Death Knight of the Undead Scourge, served his new master without question. Arthas used Frostmourne as his weapon of choice for years. Even after Arthas had become the malevolent Lich King he still used the weapon.

During the War in the Frozen Wastes, Frostmourne was used by the Lich King for many purposes, from the execution of the traitor, Drakuru, to the attempted corruption of Bolvar Fordragon. During the final battle between Tirion Fordring and the heroes of Azeroth against the Lich King himself, Frostmourne drained the souls of random heroes, bringing them into a strange realm where they met the spirit Arthas's father, Terenas Menthil II. Terenas helped the adventurers escape Frostmourne, where they continued the battle with the Lich King.

The Lich King, growing tired of the fight, shoved Frostmourne into the ground, sending a shockwave that killed all of the heroes in one blast. The Lich King then lifted Frostmourne into the air, preparing to resurrect the deceased heroes into undead champions. The Lich King gave a speech about his evil plans, but Tirion Fordring, who had been frozen in ice, freed himself and, with his holy sword known as the Ashbringer, shattered Frostmourne, freeing the thousands of souls that it had devoured over the years. The Lich King was defeated, and Frostmourne was nothing more than a pile of shards and a hilt.

Trivia

 * Frostmourne is clearly inspired by the sword Stormbringer wielded by Elric of Melniboné in the fantasy series of books by Michael Moorcock. Stormbringer is a black blade covered with runes that steals the souls of those killed by it and was featured in the book called, not coincidentally, The Stealer of Souls by Moorcock, published in 1963. Also, Stormbringer has a sister sword called "Mournblade" which may have also been the reason Arthas' sword is called "Frostmourne".
 * Frostmourne could, conceivably, have been inspired by Excalibur, the sword of legendary King Arthur. Both swords grant power to their respective wielders (whether it be political or magical), both have inscriptions upon their pedestals, and the two famed wielders, Arthur and Arthas, have similar names.
 * A hidden cave can be found in the action game X-Men Origins: Wolverine. Inside the cave, Wolverine finds a skeleton with a yellow exclamation point (Quest Avail 16x16) above his head, like a WoW questgiver, and a sword which is a replica of Frostmourne. Finding this room unlocks the "WoW!" Achievement/Trophy, depending on whether the game is being played on the Xbox 360 or the PlayStation 3. The description of the Achievement confirms the reference: "You feel cold as you examine the skeleton and read the name "Arthas" etched into the nearby sword."
 * In Guitar Hero: Warriors of Rock, both Arthas (as the Lich King) and Frostmourne (his guitar) are unlockable after earning 1600 stars and reaching level 71.
 * Frostmourne shares similarities to another blade, that is present within the game series: Legacy of Kain: Soul Reaver. Both can absorb souls, and both are wielded by nobles who were corrupted.