Frostmourne, the key that unlocked Arthas's fall. Or was it? How pivotal was the blade in Arthas's descent and subsequent rise as the Lich King? Where did this sword come from? How did it get into Arthas's hands?

Frostmourne

We do not know the exact details of how Frostmourne was made. But we do know that it was a weapon of the Burning Legion. Yes, the signature weapon of the Lich King, the lord of undeath, was forged by demons. Of course, when you consider that the Lich King himself was also a tool of the Burning Legion, and that necromancy was a form of magic that was largely first seen on Azeroth in use by the nathrezim, this makes sense. The Burning Legion made the Lich King. Therefore it's only fitting that they made his weapon as well.

It should be noted here that Tichondrius claimed at one point that the blade was forged by Ner'zhul. But this has been superceded by other sources which claim that Frostmourne predates the orc shaman. Whether this was a purposeful misleading on the part of Tichondrius, or a simple retcon, we cannot say for certain.

When Ner'zhul attempted his escape from dying Draenor through one of his dark portals, he was immedately captured by Kil'jaeden and tortured for attempting to run away. After being stripped of his body and tortured down to his soul, Ner'zhul was then ethereally bound to a set of magical armor and its accompanying weapon, Frostmourne.

Further wanting to limit Ner'zhul's ability to escape the Burning Legion's desires, Kil'jaeden encased the armor and weaponry into a block of ice from the furthest reaches of the Twisting Nether. Indeed, this was not your ordinary ice.

Kil'jaeden then instructed Ner'zhul on his new task: prepare Azeroth for invasion from the Burning Legion. The demon lord then flung the block of ice towards the unsuspecting world, where chained Ner'zhul crashed onto the frozen waste that is Icecrown Glacier.

As Ner'zhul's psychic power grew, he began to orchestrate his conquest of the frozen north. He knew that he would need a champion, and although the nerubians were proving to be able allies in undeath (at least, those nerubians he had conquered during the War of the Spider), he understood that he'd need someone greater (and perhaps, more inherently loyal) than even the powerful Anub'arak.

So Ner'zhul, now calling himself the Lich King, shoved the blade, Frostmourne, out of a crack in the magical ice and had his minions carry it far away. It was believed that the lure of such an ancient and powerful weapon would bring forth a champion strong enough to become the Lich King's next vessel.

Indeed, Ner'zhul was planning his escape.

We should stop here for a bit and discuss what Frostmourne actually is. You see, there are some misconceptions about the exact nature of a runeblade, and the powers their wield. At the simplest, a runeblade is merely a blade inscribed with runes. Nothing more, nothing less. The abilities of the weapon itself are wholly dependent upon the runes carved within and the power already imbued.

Not all runeblades are "vampiric," as Frostmourne was. Wielding a runeblade does not guarantee that you'll forfeit your soul to the Lich King. No, this was the risk encountered by anyone foolish enough to seek out a vampiric runeblade, such as Frostmourne.

Indeed, Flamestrike was a runeblade handed down through the Sunstrider Dynasty since before the highborne exile from Kalimdor. Orkenblade, a night elven blade forged specially to slay the orcs defiling their forests, is also a runeblade. Neither of these automatically allowed some far-off entity to steal the soul of the wielder.

Runeblades are also not inherently indestructible, as some people claim. There are many accounts of a runeblade being shattered. Flamestrike, again, serves as an example. It was shattered by Arthas when Anasterian Sunstrider fell defending the Sunwell of Quel'thalas. Later, it is revealed that Kael'thas fought Arthas with a version of Flamestrike that had gotten stronger since its reforging. We also have Marduk the Black and his shattered runeblade, the shards of which caused the Infectis Scar in the Plaguelands.

The runeblades that most people think of when they think of Frostmourne and the army of death knights are of the "vampiric" subtype. These are the runeblades that were forged to be conduits between the wielder and the Lich King. These are the weapons of a death knight, and are some of the most feared blades in existence. Some were forged by the nathrezim. Others were forged by powerful Scourge. And still others have perhaps entirely different origins before being subjugated by Ner'zhul's formidable will.

So with this knowledge we can gather a greater understanding of what Arthas would have faced as he claimed Frostmourne from guarding Revenants in what is now known as Frostmourne Cavern. As soon as he wielded the blade, he was willingly handing his soul over wholly to the Lich King himself.

His fall, then, was inevitable from this point.

What else do we know about the powerful weapon of the Lich King? Well, we know that it can steal the souls of the dead, as we see this when Arthas claims the spirit of Saurfang the Younger. We can also surmise that it serves as a focal point for the Lich King's powers, as witnessed when Arthas raised Sindragosa from the dead in the Wrath of the Lich King opening cinematic.

Does this weapon have an equal? Can the Ashbringer truly stand up to a sword forged in the depths of the Twisting Nether and reinforced by the sheer magnitude of the Lich King's power? We have seen Frostmourne go up against Flamestrike and the Blades of Azzinoth and survive, what other weapon can claim such a feat?

And what of Shadowmourne? What of this mysterious runeaxe said to be comparable in power to the legendary Frostmourne? Can Shadowmourne only be forged fully after Frostmourne's demise? Is Shadowmourne's creation sealed by fragments of the Lich King's legendary blade? Or is the axe a cousin of sorts—a parallel weapon created in the same vein as Frostmourne?

The events of Icecrown Citadel should reveal to us the truth.