Warning: Game of Thrones spoilers below!
For every heavy-handed Easter egg in Game of Thrones (*cough* Ed Sheeran *cough*), there are a million subtle ones that only diehard fans are typically able to pick up on. The season seven premiere is a perfect example of this thanks to a scene featuring Samwell Tarly and one very information-heavy book.
After the aspiring maester's warnings about the incoming White Walker army fall on the unbelieving ears of Archmaester Marwyn, he takes it upon himself to break into the restricted section of the Citadel's library and research ways to stop the threat on his own. While reading through a large volume (which contains some information about greyscale), the camera zooms in on a page which has a sketch of a long, curved dagger (pictured above). This isn't the first time we've seen this particular blade, but it's been quite a while since its initial appearance in the show.
Back in season one, an assassin is sent to Winterfell to kill a comatose Bran Stark in his own bed. Luckily his mother, Catelyn Stark, intercepts the assassination attempt and fights off the attacker with her bare hands. This is actually what sets off the rift between the Starks and the Lannisters, since Lord Petyr "Littlefinger" Baelish tells Catelyn that the dagger (a Valyrian steel dagger aka the "catspaw's blade") belongs to Tyrion Lannister (who actually lost it in a bet and has nothing to do with hiring the assassin). Catelyn then takes Tyrion hostage as payback without realising he's not the one behind it all, setting off the events that lead to the War of the Five Kings. Before that, though, the dagger ends up in Ned Stark's possession. So, why is there a photo of it in an ancient book at the Citadel about dragonglass?
This dagger is no ordinary weapon. After the assassination attempt, Catelyn shows the knife to Winterfell's Master-at-arms, Ser Rodrik, who reveals it's made from Valyrian steel and has a dragonbone hilt. As we're all well aware by now, Valyrian steel is one of the few things that can kill White Walkers besides dragonglass and is therefore extremely rare. We never find out who is behind Bran's botched assassination in the show, but in the books, Joffrey Baratheon is the one who sends the assassin after stealing the knife from his father. We haven't seen the knife since season one's fourth episode, "Cripples, Bastards, and Broken Things," and Ned and Joffrey are both dead, so it's unclear where the knife is at this point. If we had to guess, we'd say Littlefinger got his hands on it (in the book, it's made much more obvious that Littlefinger takes the knife, whereas the show leaves it vague).
What is clear is that the Valyrian steel dagger is obviously incredibly old, if it's in the book about dragonglass Sam finds. While reading aloud to Gilly, he notes that "the Targaryens used dragonglass to decorate their weapons without even knowing what the First Men used it for." Could it be one of the first swords ever created to take down White Walkers? Reddit user jumpman72 notes the full text next to the drawing of the dagger is as follows:
"The Valyrians were familiar with dragonglass long before they came to Westeros. They called it [some Valyrian words] which translates to 'frozen fire' in Valyrian, and eastern texts tell of how their dragons would thaw the stone with dragonflame until it became molten and malleable. The Valyrians then used it to build their strange monuments and buildings without seams and joints of our modern crafters.
When Aegon the Conqueror forged his Seven Kingdoms, he and his descendants would often decorate their blades with dragonglass, feeling a kinship with the stone. The royal fashion for dragonglass ornamentation soon spread throughout the Seven Kingdoms to those wealthy enough to afford it. Hilts and pommels were and are the most common decoration, for dragonglass is too brittle to make a useful crossguard. Indeed, its very brittleness is what relegates it to the great houses and the most successful merchants."
It seems to indicate that the dagger is made of both Valyrian steel (which we already knew) and dragonglass. Maybe what Ser Rodrik identified as dragonbone in season one is actually dragonglass.
The other significance of the dagger is that it's the same one in Arya's belt in Entertainment Weekly's promotional photos for season seven. As one of our own editors pointed out, the reasoning for this could signal Littlefinger's death at Arya's hand sometime soon. If he is in possession of the dagger and currently living at Winterfell — which is where Arya might be heading even though she states in the premiere that she's "going to kill the queen" in King's Landing — she could discover his manipulative role in passing false information to Catelyn and betraying Ned and use the knife against him. If anyone deserves to kill Littlefinger, it's Sansa, but any of the Stark children ending his life would be a pretty great full-circle moment.