A massive, impossibly deep valley north of Vad Ilghak. At its bottom the Sivakr keep a giant in perpetual captivity, chained and bled, and it is the giant's blood that pays for everything the silver elves do with memory.
Sivakr mind-magic is Deoric work, and Deoric costs life. To rewrite a memory is to spend life in the spending, and a people who edit each other as a matter of daily negotiation would burn through their own years fast. The valley is how they avoid it. Rather than pay the cost out of themselves, the Sivakr pay it out of the chained giant, which is large enough and slow enough to bleed for a long time without dying. One captive life subsidizes the whole kingdom's memory-craft. The Deoric library down in Vad Ilghak holds the texts; the valley holds the fuel they run on.
A colossal statue of Tiira overlooks the floor, the goddess the silver-elf state raised within living memory to sanctify the work done beneath her. What the statue presides over is the source of her people's power and the thing they would least like an outsider to see.
Access is forbidden to all but the highest-ranking Sivakr. Outsiders who learn what is at the bottom rarely live long enough to carry the knowledge back out.
Game mechanics
Blood sacrifice runs at roughly 40–50 HP-equivalent of life per life taken (world-systems-invariants, daemon-economics scale). The giant is not killed; it is bled continuously, which converts a one-time 40–50 HP sacrifice into a sustained trickle of life-force that funds the kingdom's ongoing Deoric memory-work. The supply is finite and tied to a single body — losing or killing the giant would strand every Sivakr memory-worker on their own life-cost overnight, which is the kingdom's single largest strategic vulnerability and the reason access is a capital secret.