"It's a death mask without a mouth."
It is one of the four Death Masks needed to release the coffin inside the underground crypt. The mask is to be placed on the statue with the "A stone statue with a hole where the mouth should be." localization upon examine.
The "Mask without a mouth" can be retrieved from the Spencer Mansion's Large Gallery after completing a puzzle involving stained-glass windows. It will be in an outside section next to the gate leading to the area behind the entrance hall.