The Lesson of Three Lights

You enter a circular chamber of black stone where cold blue light trembles across the walls. Three waist-high discs stand around a central pedestal: one brass and warm as sunlit metal, one silver and cool beneath a crescent engraving, and one of black glass scattered with tiny metal stars. Above them, a sealed stone door shows three empty sockets. Dust lies everywhere except around the discs, where dozens of old footprints end abruptly. A low chime sounds whenever anyone approaches the pedestal.