Citadel Atrium
Citadel Atrium is one of 49 beacons in Mortal Shell 2 — cleanse it and it becomes somewhere you can travel back to.
How to light it
This beacon is found on the lower level of the room past the chained platform. From Solnir boss arena, take the elevator, use the Bone Gate to cross the gap, head left where you'll see a Canyon Crawler, and walk all the way down until you reach the beacon.