When all seems lost and your power is insufficient, go to the Rock of Kuthian and speak your name to open the Vault of Souls.— Solembum
In Inheritance it was revealed that the Rock of Kuthian is a magical doorway that seals the Vault of Souls and also that it is located on Vroengard. To open it, one must speak his or her true name in the Ancient Language. Contained inside the Vault of Souls, located over a mile underneath the roots of Mount Erolas, were a cache of one hundred and thirty-six Eldunarí and two hundred and forty-three dragon eggs.
Vrael, Oromis, and their dragons Umaroth and Glaedr created this hiding place, after which the memory of the Vault as well as the Rock of Kuthian was magically removed from the minds of all who knew, including the mothers of every egg, using magic similar to that the Dragons had used in the Banishing of the Names, so that Galbatorix could never find it.
The archway above the exit had a similar enchantment that would erase the memories of the eggs from those who left it, until after Galbatorix was defeated. Moreover, the Vault of Souls is located so deep underground that magically detecting it was impossible even for Galbatorix. The Eldunarí were put in a trancelike state-almost like death-to protect them from being detected by Galbatorix and the Forsworn during the battle for Doru Araeba.
Solembum told Eragon that when all seemed lost and his power was insufficient, to speak his name to the Rock of Kuthian in order to open the Vault of Souls. The nature and whereabouts of the rock were not disclosed.
Oromis's dragon, Glaedr, also said this when Eragon later mentioned it to him. He revealed the Rock of Kuthian "struck a familiar chord in his memory", but failed to recall it. He told Eragon that he would search in his scrolls for it, but doubted he would find it written in any elven scrolls. Inheritance reveals that the Rock of Kuthian is mentioned only once in the Domia Abr Wyrda, however it contains no indication of its location or significance and its very name, like those of the Forsworn's dragons, is soon forgotten by any to whom Eragon speaks of the Rock of Kuthian. There is also only one reference to its discoverer.
Death of Galbatorix
After Eragon killed Galbatorix, the Rock of Kuthian's guarding spells ended and the memory of the Vault of Souls and its contents returned to him.
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