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The Subtle Knife is both the name of the second book in Philip Pullman’s His Dark Materials fantasy series, and the name of an object in those same books. As an object, the subtle knife is also known by the name Aesahaettr, or the God Destroyer.
Whereas the first book of the series, Northern Lights — The Golden Compass — in the United States, followed the story of a girl called Lyra in a world very different from our own, the second book begins in our world, following a boy named Will Parry. Will stumbles through a window into another world, and finds himself in Cittagazze. Here he meets Lyra, and their story continues together. The two go back to Will’s world and learn more about the mysterious Dust, and its relationship to quantum physics.
The two return to Cittagazze after Lyra has her alethiometer stolen, and there Will is made the bearer of the subtle knife, which is capable of cutting portals between worlds. The story continues, following other characters from the earlier novel as more of the prophecy surrounding Lyra is revealed, and the plot against Heaven thickens. The book ends with Will discovering Lyra missing, and being taken under the wing of two angels.
The subtle knife itself, Aesahaettr, was made by men from the world of Cittagazze in ages past. The knife has an edge of incredible sharpness, capable of cutting through even steel as though it were paper. It also has another edge, with a sharpness far beyond anything on the gross physical level, capable of cutting through the very fabric of space to open holes to other universes.
The subtle knife is also known as the God Killer, because its sharpness allows it to cut through any substance, even threatening the Authority Himself. It is for this reason that the Magisterium attempts to capture the knife from Will, to protect the Authority from its misuse, and why Lord Asriel needs it to win his war. It is revealed that the main reason the first War in Heaven failed was that none of the rebelling angels were capable of harming the Authority.
When used without the proper concentration, the subtle knife shatters, and it is broken this way by Will at one point unintentionally. After much persuasion, the armored bear Iorek Byrnison agrees to reforge the subtle knife. When deciding, however, he probes the nature of the knife with his deep empathy for metal, and tells Will that it has its own desires and will, and that these were not necessarily positive.
The people who made the subtle knife, the philosophers of the Torre degli Angeli, used the knife to travel to many other universes. Often they went in search of new wisdom or technology to enrich their own lives and that of their world. Although they were supposed to seal each rift they cut after they cut it, occasionally they forgot to do so, leaving connections between universes, such as the one Will discovered in his world.
Each time the subtle knife was used in this way, it released a creature of darkness, a Spectre, from the Abyss. Additionally, when holes were left open between worlds, the sustaining Dust was drawn to those holes, ultimately leaking out of the worlds and causing great ill. This is why ultimately Will had to close each hole and destroy the subtle knife, to ensure that it could never be used again, to preserve the Dust in the world.