These richly decorated hallways originally seemed to lead to the Grail sanctum, but as Aeternis was transformed according to Tobias' vision, it was turned into an access hall to the face of Baphomet. Later on, it became a theo nest, with a portcullis trap to ensure that none who aren't suited to hold the Grail may pass.
This is a two-chamber area. The first one contains a sleeping theo, which you may avoid by hugging the left wall or simply gunning down with the MS-7 Sniper. In the corridor beyond is Colin Scott's unfortunate corpse and Shield IV. Use the lever before crossing under the portcullis to avoid sharing his fate. Talk to Scott after picking up the shield, then head to see the face of Baphomet.
- Shield IV: By Scott's corpse.