This is the crypt beneath the Cathedral, used to bury the dead members of the Templar detachment at Aeternis.
This small room contains two caskets on the floor and a relief of a knight in the background. The relief is actually an intricate safe and in order to open it, you need:
- the hilt from Cobweb's lair,
- the shoulder from the clock,
- the ring from the poison gas room,
- the dagger from the perilous chapel, found on the beams.
Use all of them on the appropriate places on the statue, completing it according to the knight relief diagram. The reward? A ton of intel.