Candelum is an Aureum sun-work, and it is the wrong one to have survived. The Aureum cut their great intake-vaults into the high crossings where Bryn's band ran over the peaks, polished hall-wide mirrors, and ran the captured light down a mile of shaft and glass channel into the deep. Candelum was none of that. It was a distribution hall far down one chain, a place where a thinned thread of light was split and parcelled out to the working levels below it. The Aureum named their intake-vaults the way you name a cathedral. This one they called the little candle, because next to the vaults above it that is what it was. For the full account of how the Aureum carried daylight underground, see the Aureum racial entry.
When Bryn's path drifted off the old crossings roughly a century before the present, the vaults at the head of every chain went dark, and that is the canonical end of the Aureum as a civilization. The mirrors still turned over nothing. The channels ran dry from the top down. Looters and remnant glass-tenders stripped the famous halls of anything that still held a charge, because everyone knew where the light had been kept.
No one bothered with Candelum. It was a tail-end splitting hall in a played-out section, and when the thread feeding it failed, whoever sealed the level sealed it as an afterthought. What that afterthought trapped was the reserve already inside the hall: tier on tier of stoppered glass holding decanted sunlight, the slow golden fluid the Aureum drew off their prime mirrors and thickened for storage. The bonfires went out. The candle, sealed and ignored, kept.
The lower gallery of Candelum. Six tiers of glass cradles run the length of the hall, each cradle holding a stoppered vial as long as a forearm, and inside the vials a fluid the color of poured honey lies so still it looks solid. One channel at the far end still runs, a thread of light no thicker than a wrist, feeding a single decanting basin where the Tenders draw the day's ration. Everything the light touches throws a hard noon shadow. Everything it does not touch is black.
What remains is the only working light the Aureum have left, and it is a finite tank with a measured tap. The Last Lamp resistance found Candelum, holds it, and rations what comes out of the basin, because the day the glass runs empty is the day they have nothing to defend. The hall is not on any imperial map and has no surface entrance worth the name. Reaching it means going down through levels the Bloodlings hunt, which is the other half of the problem: the Bloodlings know Candelum is down there, and they want what is in the glass for reasons of their own.