Once you’ve got the diving mask and used your new-found underwater breathing skills to drain the water out of Dunehollow, thus completing the Sea of Sands main quest in Horizon Forbidden West, you’ll be able to start the Night of Lights errand quest by talking to Stemmur at the Hidden Ember settlement. This quest takes you back into Dunehollow, only this time it’s dry.
You’re looking for a “gizmo” in an underground ruin, but once you’ve defeated the machines and found the underground ruin, you’ll discover that the gizmo is behind a code-locked door. Go upstairs from the door and use your Focus to find a Datapoint called Malfunction. This Datapoint includes the code, but the data is corrupted, so you only get the last three digits: 135.
Related: How to find The Daunt Relic Ruins passcode and key in Horizon Forbidden West
Head north into the fenced area, then jump onto the box in the northeast corner. Turn to the west and use your Pullcaster to pull the top crate towards you. Now grab the bottom crate and drag it over to the angle in the ramp by the west wall. Use your Pullcaster to pull the other crate back again, so that you can drag it out, then push it up the ramp and onto the first crate. Now drag them both over to the fence. Before you use the crates to climb up and through the gap in the fence, shoot your Pullcaster through the hole in the fence and pull the crate next to the vent out of the way. Now you can go inside the fenced off corner and pull the vent cover off with your Pullcaster.
The vent will lead you outside where there’s a metal camp that also needs the Pullcaster treatment. In the next room you’ll find the RSVP Requested Datapoint, which contains the first half of the code: 739. So, the Night of Lights door code is 739135. Use that code to open the door and get the gizmo (or the Ornament, if you prefer).
