'The Vault of Souls was an unknown location. Solembum, the werecat, told Eragon that if he was ever in need of strength, he should go the Rock of Kuthian and open the Vault of Souls by speaking his name.
Oromis stated in Brisingr 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.' COURTESY of http://inheritance.wikia.com/wiki/Main_Page.