In "The Spoils of War," the fourth episode of Game of Thrones' seventh season, Littlefinger presented Bran with a curious gift: the dagger someone once tried to kill Bran with way back in Season 1. As far as housewarming presents go, that's an iffy one; Littlefinger probably should have stuck with a bottle of wine or some nice housewares. But there's no denying the dagger was a significant gift to give, and it becomes even more so when you take a closer look at the journey it has been on. The history of Littlefinger's dagger on Game of Thrones reveals how important it may be in the future.
The first time the dagger was seen on the show was during the attempt on Bran's life by a hired assassin in Season 1. After stopping it with her bare hands, Catelyn Stark took ownership of the dagger and then brought it to Ned in King's Landing. That was where Littlefinger entered the story: according to him, he owned the dagger previously and then lost it to Tyrion in a bet, but that ended up being a lie intended to implicate Tyrion in the attack on Bran. He then used it against Ned just prior to his imprisonment (and later beheading). Littlefinger has held onto that knife since, though in Season 7 Episode 4 it quickly passed from him to Bran to Arya.
George R. R. Martin's books are slightly more illuminating about where the dagger came from, as is to be expected. While its history is mostly the same, Littlefinger actually lost it in a bet to Robert, not Tyrion. That was where it remained until Joffrey got a hold of it. Both Tyrion and Jaime came to believe that Joffrey stole the dagger from his father and hired the assassin who tried to kill Bran, though that was unconfirmed at the time of Joffrey's death. As for his motive, it appeared Joffrey just wanted to impress his dad in the most twisted way possible. So, classic Joffrey.
But the dagger's past may not be as important as its future. It's made of Valyrian steel, which means it's a very useful thing to own in a world where that's one of three substances that can kill the oncoming White Walkers (the other two being dragonglass and dragon fire). An illustration of it also showed up in one of Sam's books at Old Town, so the foreshadowing of its significance has been heavy.
It's uncertain exactly what part the dagger will play in upcoming episodes, but fans would do well to keep an eye on it. It will almost definitely make another appearance — and perhaps even take someone's life when it does.