The Lady's Quarters
Head to the right and up the stairs, then enter the bedroom.
Go all the way to the left, then climb onto the table and push the vase off to smash it. Grab the key from the pile of broken pieces.
Climb onto the dresser in the next room and smash the final Statue (1/1), to unlock the Rascal Trophy or Achievement.
Carry the key downstairs to unlock the door on the landing.
Keep heading right, and be ready to run when the priestess appears.
Go to the room on the far right and pick up the mirror from the dresser, then carry it back to the room on the left.
Keep dashing into the middle of the circle of light, then pointing the mirror at the priestess when she attacks.
Eventually she'll be defeated, at which point you can go over and interact with her.
For the next scene, simply walk forward to the end of the corridor and you'll have completed the main game - congratulations!