Spoilers below for those who haven't yet watched Game of Thrones' return for Season 7.
During one scene in its shock-filled Season 7 premiere, Game Of Thrones referenced something we haven't seen since Season 1, and fans are racking their brains trying to figure out why. The scene in question showed Sam Tarly in the Citadel, reading a page out of a stolen book that showed the noteworthy Catspaw Blade, the same one used in the assassination attempt of Bran Stark, at which point it was taken to King's Landing by Catelyn Stark to find the party responsible. Here's why fans should be interested in this seemingly random item, and what it could mean for Season 7.
This is actually the second time Game Of Thrones has teased that particular dagger in regards to Season 7. The dagger was also seen on the hip of Arya Stark in a promotional photo shoot for the series, which is obviously weird since she wasn't in possession of it before, so where did she get it? In Season 1, the dagger is last seen on Ned's desk, so it could still be in King's Landing. Of course, it could also be on the hip of another major character who presently owns the dagger in the novels: Littlefinger. But with both him and King's Landing in opposite directions of our Stark girl, how would she obtain the weapon?
If the dagger is in King's Landing, the answer is simpler. Arya is on her way to kill Cersei, and regardless of whether she is successful there or not, she can certainly steal the Catspaw for the simple fact that it reminds her of her father and family. If the blade is with Littlefinger, then that's a completely different story, and we'd be wise to look for Petyr Baelish to potentially encounter Arya in a reunion situation he'd rather not be in, with the Stark child exacting revenge with the same dagger used to start this whole Game Of Thrones mess.
Another interesting fact about the blade is that it is made of Valyrian steel. If killing Baelish or Cersei is not in the near future for Arya -- or even if it is -- her possession of the blade gives her a useful weapon in the fight against the white walkers should she cross paths with them. With Arya heading south, an encounter with that deadly army isn't so likely right now, but down the road, it seems inevitable that she'll come in contact with them, and maybe more.
Going back to just the Catspaw Blade, whoever is in possession of it could be outed as the true mastermind behind the plot to kill Bran Stark. Littlefinger lied to Catelyn initially and said the dagger was his, but that he lost it to Tyrion in a wager. Tyrion would later dispute that claim, and it's pretty clear Littlefinger lied, but why? Did Littlefinger take it upon himself to escalate the situation and allow everyone to think it as a Lannister plot, as many have surmised? It sounds sinister and devious, and that's Littlefinger in a nutshell.