At the end of a Nightmare Containment activity, two of five specific doors in the Derelict Leviathan’s Castellum area unlock. Behind them is a treasure chest that has a chance to reward an Opulent Key if you don’t already have one in your inventory. You will know which door’s open as they have a red-lit switch in front of them when not active. You have about two minutes to open the two doors before the lockdown resets, and you’ll need to complete another Containment to unlock them again.
Here are the locations of the five Nightmare Containment lockdown doors.
Nightmare Containment lockdown chest 1: Left of the Final Nightmare
The first of two easy-to-check chests is just down the hallway on the left of the final boss arena. It’s closest to the back wall of the Castellum.
Nightmare Containment lockdown chest 2: Right of the Final Nightmare
The second easy-to-check chest is down the right hallway closer to the wider Castellum.
Nightmare Containment lockdown chest 3: Near the initial spawn point, in a corner
From the final boss arena, head across the Castellum until you almost reach the spawn-in point. Facing the spawn room, look to the right, and you’ll see a small nook with a lever door at its back. This is the third lockdown chest door.
Nightmare Containment lockdown chest 4: On the lower level directly opposite the third lockdown chest
Using the third lockdown chest as a guide, take the stairs down to the lower area that used to lead to the Gauntlet encounter in the Leviathan Raid. About halfway down, a short, blue-lit hallway will be on your right. At the hall’s end is the fourth lockdown chest door.
Nightmare Containment lockdown chest 5: On an upper platform near the Pleasure Gardens entrance
The fifth and final lockdown chest door rests on a platform up and to the left of the entrance to the Pleasure Gardens area from the Castellum. If you use the fourth lockdown chest as a guide, take the stairs at the opposite end of the lower area, then turn around until you reach the Pleasure Gardens entrance and its many columns. Facing the entrance, look up and to the left. There is a platform you can jump to containing the last lever and lockdown chest door.
