- "It's mine."
- ―Petyr Baelish
After the assassination fails, Catelyn shows the dagger to Maester Luwin, Ser Rodrik, Robb Stark, and Theon Greyjoy. They reveal that it is an elaborate dagger with a dragonbone hilt and a Valyrian steel blade.
Catelyn resolves to carry the blade to her husband in King's Landing to investigate further. Varys and Petyr Baelish both examine the blade and Littlefinger claims that the dagger belongs to Tyrion Lannister.  After he is kidnapped, Tyrion denies that, but Catelyn does not believe him.
While reading, Sam lingers on a page detailing the use of dragonglass to decorate weapons. The page contains a drawing of the Valyrian steel dagger:
The Valyrians were familiar with dragonglass long before they came to Westeros. They called it "zīrtys perzys" which translated to “frozen fire” in Valyrian and eastern tales 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 building without seams and joints of our modern castles.
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 affod it. Hilts and pommels were and are the most common decoration for dragonglass if too brittle to make a useful crossguard. Indeed, its very brittleness is what relegate it to the great houses and the most successful merchants.
Littlefinger later gives the dagger to Bran Stark. Bran, however, is now the Three-Eyed Raven and incapable of fighting on his own, so he gives the dagger to Arya Stark, in the presence of Sansa Stark. Sansa notes that Littlefinger never gives away anything for free, and suggests that he must have some sort of ulterior motive. Later, Arya uses the dagger when sparring with Brienne of Tarth. 
|Season One appearances|
|Winter Is Coming||The Kingsroad||Lord Snow||Cripples, Bastards, and Broken Things||The Wolf and the Lion|
|A Golden Crown||You Win or You Die||The Pointy End||Baelor||Fire and Blood|
|Season Seven appearances|
|Dragonstone||Stormborn||The Queen's Justice||The Spoils of War||Eastwatch||Beyond the Wall||Episode 67|
Behind the scenes
- In the June 2017 issue of the Entertainment Weekly Arya Stark was on the cover with the dagger on her belt. This was somewhat of a spoiler that the dagger would reappear in the TV series.
- Actor John Bradley-West (Samwell Tarly) explained in interviews that in the Season 7 premiere, when Sam is reading a book, he was specifically instructed to linger on a detailed sketch of the Valyrian steel dagger - as this was apparently intended to remind the audience about it: "When we were shooting that scene, I was literally told 'make sure that you linger on this page.' They were shooting over my shoulder and said make sure to linger on it. Make sure we get a good shot of this page before you turn the page over."
In the books
Similarly to the show, Catelyn shows the dagger to Littlefinger and Varys. Littlefinger hands it to Ned Stark and advises him to throw it away.
Later, Ned examines the dagger, trying to figure why Tyrion or anyone else would want Bran dead. He has a strong (but incorrect) feeling that the dagger and Bran's fall have something to do with Jon Arryn's murder.
While Robert is lying near death, Ned considers what he should do. Littlefinger advises him to make peace with the Lannisters. Ned firmly refuses; to emphasize his stand, he places the dagger on the table, a reminder that the Lannisters sought to murder his son.
During the brawl in the throne room, Littlefinger slides the dagger from its sheath and shoves it up under Ned's chin.
Unlike in the show, Catelyn reveals to Tyrion that it was Littlefinger who told her the dagger belonged to him. Later she discovers that Littlefinger lied to her, but it is too late to undo the damage she caused by kidnapping Tyrion.
Some time after Tyrion returns to King's Landing, he chats with Littlefinger, commenting about his dagger. Littlefinger draws the dagger, glances at it casually as if he had never seen it before, then says with mischief in his eyes: "Valyrian steel, and a dragonbone hilt. A trifle plain, though. It's yours, if you would like it". Tyrion muses "He knows, the insolent wretch. He knows and he knows that I know, and he thinks that I cannot touch him".
It is unknown what has become of the dagger afterwards; Littlefinger presumably still has it in possession. In "A Storm of Swords" it is mentioned that Littlefinger carries a dagger and uses it to cut food, but it is not specified whether it is the above mentioned dagger.