The Emperor's Vaults, also referred to as the Immortal Emperor's Vaults, were a series of mysterious vaults containing Tenebrae's dark relics that even he feared were too dangerous and powerful to use.
After constructing them, Vitiate sealed deadly guardians inside the vaults to protect the vault from intruders, where they hibernated until disturbed. Its existence was kept a closely guarded secret, so much so that its details were nothing more than a rumor among the Sith Empire's leaders. Likewise, the Republic Strategic Information Service had very little knowledge of the vaults, with expeditions to locate a vault turning up nothing, such as with Jen Alkier's hunt.
The existence of such vaults were revealed to the Alliance Commander when the Commander sought refuge in one at the Sanitarium during the Commander's hunt for Vaylin. The Commander found and retreived Dramath's holocron from the vault. However, this vault and the rest of the artifacts within were destroyed along with the Sanitarium during Vaylin's ritual.
Zildrog, in a vault.
Following the Sanitarium's destruction, Gault Rennow hoped to find other vaults filled with priceless artifacts that he could sell on the black market. Gault attempted to reach out to an old exiled Sith contact who once mentioned the existence of such structures after a few too many Tarisian ales, a statement Gault dismissed at that time, but Gault found out that she vanished mysteriously a few days after she told Gault of those vaults.
During the Commander's conflict against the Order of Zildrog, the Commander stumbled upon another vault under an ancient Sith Temple which housed Zildrog, whom Vitiate had deactivated after his Ritual of Nathema many years ago.
The Emperor's Vaults first appeared in Star Wars: The Old Republic: Knights of the Eternal Throne during Chapter VII: Into the Void.