The locked door in Glenbright Manor is opened with the Glenbright Basement Key. According to the walkthroughs, you first need to find the hidden passage under the staircase on the first floor, then take the key from the secret room and use it on the basement door.

Steps

  1. Go to the first floor near the staircase.
  2. Find the painting of two soldiers on horseback under the stairs.
  3. Walk into that wall to reveal a hidden passage.
  4. Pick up the Glenbright Basement Key inside.
  5. Return to the basement and open the locked door with that key.

Extra context

The basement itself is reached through the fireplace puzzle: the two daggers are placed into the fireplace in the office to reveal the secret room and ladder down.

Multiple guides agree that normal keys do not work on that basement door, so the special basement key is the intended solution.

Note

If the door still refuses to open, one discussion thread suggests the manor’s hidden wall can be finicky, so re-approaching the secret wall or reloading may help.

What you needWhere to get itWhat it opens
Glenbright Basement KeyHidden room under the staircaseLocked basement door
TL;DR: open the hidden wall under the stairs, grab the basement key, then use it on the locked door.