The Celestial Merchant is a special kind of Shop who (like the Mysterious Merchant appears at random whenever you complete an Exploration Zone. This seems to be more common when you are blitzing the zones.
She will always have a few items for trade:
- Light Envoy soulstones for 25 hero soulstones
- 20 Medium Runestones for 10 hero soulstones
- Amethyst (worth 100,000 gold) for 10 soulstones
You can use Diamonds to refresh her stock to either reset the persistent stock items for sale or to get other random items