Æsahættr (pronounced AS-hattər [ˈæs.hætə]), also known as the Subtle Knife, is a knife with a blade so fine it can rip through the fabric between the worlds of the multiverse.
A knife with a double-edge, the Æsahættr possessed one edge of keen steel, which could cut through any physical material. The other edge was even sharper, extending to a point so fine that the eye could not see it. This edge was able to search out gaps in the fabric of space and thus cut through the fabric of the universe and into others, referred to in the books as other worlds. No known material can resist the knife, although it will break should its wielder attempt to use it whilst thinking about a person they love. The characters believe that this is because the knife is incapable of cutting through love.
The knife was made by philosophers of Cittágazze, from the Torre degli Angeli, who cut many windows in their world. It was won in a fight by Giacomo Paridisi, who lost the two small fingers on his left hand obtaining it, and became the bearer. He hid in the Torre degli Angeli from the insurgence of Spectres in his world, for many years protecting the knife. A young Cittágazze man named Tullio tried to take it from him, but Will Parry and Lyra Belacqua had been sent by Lord Boreal to possess the knife, and Will fought Tullio, prevailing, obtaining the knife, and losing the same two fingers in the process—marking him as the new bearer.
Lord Boreal and the Magisterium of the Church wanted to possess the knife because its ability to cut through any material threatened their Authority, who had survived the First Rebellion only because the rebel angels could not harm him with any existing weapon or material at the time (their fears were later confirmed when Will and Lyra used the knife to free the authority). Will and Lyra instead took the knife and used it to travel between their worlds for a long while, though the knife once broke when Will, trying to open a window, was distracted by the thought of his mother. The shattered pieces were collected by the two children, though, and repaired by the armoured bear and great metalworker Iorek Byrnison.
Iorek warned Will and Lyra that the knife had purposes of its own—indicating an intelligence akin to that of the alethiometer. He also indicated these purposes might not be good. Near the conclusion of the trilogy, the angel Xaphania explained that Dust had been leaking out of the worlds through the windows made by the knife, and that each time the knife made a window, it drew up a Spectre from the Abyss. Thus all windows leading between worlds had to be closed by the angels, except for one. Will and Lyra initially believed this one window could be between their worlds, but then realized that they had made a promise to the shades in the world of the dead: to leave them an opening so that they can leave it and rejoin the universe, by emerging into the world of the mulefa. Thus, Will realized he had to shatter the knife. As people in the trilogy's fictional universe are not capable of surviving for long outside their own universes, it was also necessary that Will and Lyra part.
Asahættr literally means "God-Destroyer".