Shougi Board, and Underground Ruins

Touya and the others head to ruins on a new quest from the Guild to defeat a dullahan. There they encounter the ancient royal capital, abandoned after the capital was moved a millennium ago. After finishing their quest, Touya and the others explore the underground ruins for treasure. Yet what they find isn't a mountain of treasure, but a magic beast made of crystal! No attacks work on its hard body, it absorbs magic, and it even regenerates from wounds. Touya and the rest struggle against such a powerful creature... What on earth is the true identity of such a magic creature?!