The Sword of Aether is a flat, silver metal hook that takes the place of Marethyu's left hand.
It is thought that Aether magic may be a combination of all magics as Marethyu's hook was made when all Four Sword of Power were combined. It is possible that if you combine the elements in this order you will get Aether magic: Fire (Clarent), Earth (Joyeuse), Air (Durendal) and finally Ice/Water (Excalibur).
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In The Enchantress, Josh Newman is left behind at the Pyramid of the Sun to complete the prophecy. He combines Clarent, Joyeuse, Durendal, and Excalibur into Aether and destroys Danu Talis in order to fullfil the prophecy.
Apart from The Enchantress, Aether makes only minor appearances.
Curiously, although stated that only 'two that are one', after learning all Five Elemental Magics can unite Four Swords into Aether, it was Josh alone that united them. This could be a goof due to Josh not mastering Earth and Aether Magic. This could also be metaphorical, since Clarent and Excalibur are, in fact, 'two that are one'. It is possible that in process of fusing swords, Excalibur and Clarent('two that are one') fused first, thus allowing Joyeuse and Durendal to fuse along for the first time ever. This also explains why Air Sword and Earth Sword never fused together, although they stood near each-other for years. Or the 'two that are one and the one that is all' could be considered to be just the Sword of Aether, because it is the combination of all magical elements that would mean that it is its own twin, and since Aether is the combination of all the magical elements which make up the world, that could make the Sword of Aether the 'one that is all'