Found: YA Fantasy: Prince restored to power

I don’t remember any names, but the book opens with a healer and his son that treat prisoners working in a mine, and one of the prisoners turns out to be the prince that everyone thought was dead. He has a magic tattoo placed there by a blue mythological creature, and later the creature appears and proves the prince's identity.

This is definitely Beyond the Hanging Wall by Sara Douglass. The creature doesn't place the tattoo though. The heirs to the throne are tattooed with ink mixed with the creature's blood which gives them a link to it and they have to answer the creature correctly to be confirmed as the rightful heir.

>2 Nerilka: Thank you!!!!