Spoiler Alert: This piece may contain spoilers from any material of A Song of Ice and Fire.
It's interesting to observe that, until a year ago, many people were on-board with the idea that Longclaw was actually Blackfyre, one of two known ancestral Targaryen swords. Then, George had to go and ruin everyone's fun when someone asked him about it on his blog:
Fan 1: "I'm a big fan of your work and have a question for you: Is Longclaw really Blackfyre, the bastard sword of the Targaryen kings? Searching through the books I found the description of both blades to be almost identical."
Fan 2: "I think most people speculate that Longclaw may actually be Darksister. The sword brought to Castle Black by Bloodraven. Any truth to that?"
Fan 3: "Is there something hidden underneath the crypts of Winterfell?"
GRRM: "Hey, guys... enough of this. I am not going to reveal anything in the comments section of my Not A Blog. That's what the BOOKS are for."
Notably, this exchange is reminiscent of the time George dodged the question of Rhaegar's burial. George likes to answer questions as long as people don't get too near the truth of the secrets he wishes to remain hidden for now.
Therefore, we face the question: why not Dark Sister? The primary objection to the Darkclaw theory is a great litmus test for determining which fans are RPG gamers and which fans are historians. Those who believe ASoIAF is a D&D rulebook will declare that Longclaw cannot be Dark Sister because a bastard sword cannot be a longsword. I like a good RPG as much as the next guy, but that's not what George is writing. For those who would like clarification, this handy primer from ARMA describes why it is perfectly acceptable to call a bastard sword a longsword. This is especially true if the hilt was remade because The term [bastard sword] may derive not form the blade length, but because bastard-swords typically had longer handles with special 'half-grips' which could be used by either one or both hands.
|Left: Dark Sister by Velvet Engine from The World of Ice and Fire.|
Right: Longclaw from Jalic, Inc's official Valyrian Steel blade line.
The relative dimensions are in no way contrary to canon.
The pommel was a hunk of pale stone weighted with lead to balance the long blade. It had been carved into the likeness of a snarling wolf's head, with chips of garnet set into the eyes. The grip was virgin leather, soft and black, as yet unstained by sweat or blood. The blade itself was a good half foot longer than those Jon was used to, tapered to thrust as well as slash, with three fullers deeply incised in the metal. Where Ice was a true two-handed greatsword, this was a hand-and-a-halfer, sometimes named a 'bastard sword.' Yet the wolf sword actually seemed lighter than the blades he had wielded before. When Jon turned it sideways, he could see the ripples in the dark steel where the metal had been folded back on itself again and again.
You’ll want to wear that over the shoulder, I imagine. It’s too long for the hip, at least until you’ve put on a few inches.
Slung across his back in a black leather shoulder sheath was Longclaw, the hand-and-a-half bastard blade the Old Bear had given him for saving his life. A bastard sword for a bastard, the men joked. The hilt had been fashioned new for him, adorned with a wolf's-head pommel in pale stone, but the blade itself was Valyrian steel, old and light and deadly sharp.
"He let him feel the lightness, the balance, had him turn the blade so that ripples gleamed in the smoke-dark metal. "Valyrian steel," he said, "spell-forged and razor-sharp, nigh on indestructible."
Longclaw was feather-light.
Notice that while it is longer than the practice swords Jon is used to, it is short enough for a grown man to wear at the hip.
Now for Dark Sister's descriptions:
the Valyrian longsword Dark Sister, whose slender blade is designed for a woman's hand
Blackfyre is "somewhat larger and darker" than Dark Sister or Lady Forlorn.
The word "slender" is a reference to thickness or width, not length. The fact that Blackfyre is "somewhat larger" may refer to length or width or both, but is ambiguous. Ultimately, we have no reference for how the length of Blackfyre compares with Longclaw. Nor do we have a reference for Longclaw's width. Therefore, there is no point at which the sizes of the two swords contrast.
Notice, also, that Longclaw's lightness is highlighted ad nauseum. This is consistent with a Valyrian blade "designed for a woman's hand." The slenderness of Dark Sister and Lightness of Longclaw are also quite consistent with the physical description of Snow himself! Jon was slender where Robb was muscular, dark where Robb was fair, graceful and quick where his half brother was strong and fast.
Observe the artistic representations of Dark Sister and Longclaw in the image and how similar the blades are, especially the three fullers in both. Dark Sister's image is from The World of Ice and Fire. See Ran's comment in this link regarding the canon of the book's illustrations. Of Jalic, Inc's representation of Longclaw, George has this to say: "Chris and I have been trading emails for months, fine tuning the design, and we finally have one we love." This certainly is not proof, but it is an apt demonstration of how they may look alike.
There is one time that the word "great" is used in reference to Longclaw: Longclaw was slung to his saddle, the carved stone wolf's-head pommel and soft leather grip of the great bastard sword within easy reach. Even if this did refer to a great size, it's quite ambiguous. Consider, however, that the only other times the words "great bastard" are mentioned in any canon texts they refer to Aegon the Unworthy's sons, and one of them is the last known wielder of Dark Sister.
This brings us to the biggest piece of evidence that makes Dark Sister a more likely fit for Longclaw than Blackfyre: Bloodraven. He was a Lord Commander of the Night's Watch before he became a tree, and it's not unlikely that he left Dark Sister in what became Mormont's chambers. It is widely believed that Bloodraven communicates through Mormont's unusual raven, which, as it happens, loves to perch upon Longclaw. It also seems that they both gave Jon the blade:
"The fire melted the silver off the pommel and burnt the crossguard and grip. Well, dry leather and old wood, what could you expect? The blade, now... you'd need a fire a hundred times as hot to harm the blade." Mormont shoved the scabbard across the rough oak planks. "I had the rest made anew. Take it."
"Take it," echoed his raven, preening. "Take it, take it."
Finally, here's another subtle nod to Dark Sister, sword of the famous Dragonknight, Aemon Targaryen: The gift of a sword, even a sword as fine as Longclaw, did not make him a Mormont. Nor was he Aemon Targaryen.
It seems that Mormont is fibbing just a bit. Why? The man certainly demonstrates a notable understanding of Targaryen history in Jon's first chapter of the second book. Could he be in on Ned's conspiracy? I suspect that the history of House Mormont has been fabricated along with the history of its ancestral sword, but that is a story for another time.