The relics of Dragon Five Island were not located on the surface but underground. Unlike the Renewal Demon Clan who worshipped the Sun, the Primordial Demon Clan revered the Earth—or more precisely, the planet itself. Based on the available data, the important relics of the ancient Primordial Demon Clan were all underground, and their structures resembled inverted regular tetrahedrons. Generally, their buildings were topless, bearing some resemblance to Ancient Greek theaters or the Roman Colosseum—only triangular in shape. Perhaps the triangle held a different significance in their culture.
The relics on the Dragon Flow Islands might have been built to seal away the strange force or cataclysmic phenomenon known as the "Eternal True Color" and thus were established in the dark depths of the underground. Those ancient Demon Beasts hoped to rely on the planet's powerful strength to forever seal those things.
