Vell, Immortal Life

On the last page of History of the Ocean, there is a phrase that looked as if it was written in a rush. "The Immortal Sea Monster"

It refers to Vell and its ability to regenerate its heart. According to the diary left by an alchemist who once used the Vell's Heart to create an alchemy stone, Vell is still roaming the sea even after he took its heart. This remains an unsolved mystery.