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
Oromis stated that the Rock of Kuthian struck a familiar chord with him, but he had never before heard of the Vault of Souls. Also, Arya had stated that it sounded familiar, but could not place why.
Eragon found the location to the Rock of Kuthian in the book Domia abr Wyrda. He traveled there with Saphira and the Eldunarí of Glaedr and the three realized that they were required to speak their true names to gain entrance to the Vault of Souls, which both Eragon and Saphira had to discover for themselves.
Inside the Vault
The Vault of Souls contained Cuaroc and 136 Eldunarí including that of Umaroth, Vrael's Dragon, as well as 243 dragon eggs. Save for 26 of them that were set aside for the Riders, the eggs were mainly those of wild dragons. One egg, a red and gold, was nearly five feet tall. When Vrael, Oromis, and their dragons created this hiding place, 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. Also, the Vault contained a pit that opens to a lake of molten stone (magma). This lake provides heat for the eggs and light for the Eldunarí to maintain their strength.
The Eldunari, at some point, contacted the werecats and told them to tell someone who had the potential to defeat Galbatorix that if they needed to find a weapon to look under the Menoa Tree and if all seemed lost and they felt powerless to go to the Rock of Kuthian and speak their name to the Vault of Souls, however the memories of this conversation was removed afterwards leaving the message in the werecats' minds.
When Eragon entered the Vault of Souls, he left with all but five of the Eldunarí contained therein – those five remaining behind to guard the eggs. These 131 were far inferior in number to the countless Eldunarí captured by Galbatorix. Nonetheless, their combined might was enough for Eragon to defeat the evil King at last by forcing him to experience all the feelings he had caused in people since he was born. This was too much for him to bear and he killed himself in an enormous explosion. Eragon and Saphira eventually returned to the Vault to move Cuaroc, the remaining Eldunarí and all the eggs to a land east of Alagaësia to rebuild the Dragon Riders.
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