Thrall Artifact
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The Thrall Artifact is discovered during Beneath the Ashes. It is a rectangular-shaped stone, inscribed with blue glowing letters which apparently spell out the magic words "Och Eshivar". After these two words are spoken, Lara has control over the Thralls.
It is also suggested that Lara can control the Doppelganger with the magic words; this may be clarified in Lara's Shadow. It is worth noting that "Och Eshivar" is "ravish echo" written backwards, the "echo" perhaps being the Doppelganger, an echo of Lara.