A Dungeon wasn't an unfamiliar term to Hugo. He'd visited one when he was 13 and Gaia was still in his crazier years. The few days he'd spent in the dungeon led to the birth of one of his many achievements; killing an SS-rank monster.
He hadn't killed the monster on his own but had some help from Gaia because he was at the time, too weak.
Even so, the aura of the monster lying within the depths of that cave had surpassed the level of the SS-rank monster he killed so many years ago. It probably had to do with the fact that it was a bipedal monster. Bipedal monsters often had more power than other kinds. The monster he'd faced back then wasn't. That one was a massive wolf that could call lightning to its aid.