Well, I just read that page (
http://www.gotoreads.com/high-king-montival-novel-change?page=0,237)
The sound grew, faint and haunting, like the bridle-bells of the Fair Folk heard through trees on a moonless night. Then it rose to a peal like a silver carillon, and white light flooded down the stairwell. The skin at the back of his neck crawled a little in awe, and he heard whispered prayers and saw signs made.
“The jewel in the Sword. It must be as bright as the Sun itself,” he whispered, and then it died away.
Artos has trust his (magic?) sword in front of the door of the bedchamber. After they start having sex, it starts to ring, and a "white light flooded down the stairwell", presumably from the jewel in the hilt, thus the remark.