Rein had never seen the great monsters that he needed to defeat to clear this world's trial. But he still did a lot of research on it. For that reason, he knew a lot about this monster. That was why he had been trying to find some traces of this being anywhere in this world.
From what Rein was able to guess, this monster was a prehistoric monster or even the first ever monster who had once roamed the surface world. Then for some reason it fell to the depths of the underground and got trapped in the core of the world. Rein still had no idea about why or how that happened and he was trying to find that out.