When they heard Oliver's story, Andrew and the others were pale with fright, because they couldn't believe how dangerous the abyssals were. Oliver, serving as general of the army of the kingdom of Numa, had access to information that ordinary people did not have, so he knew everything that was known about the abysmal.
For this reason, his words carried so much weight. Most people knew of the existence and danger of the abysmal, but everything was based on legends passed down from generation to generation. However, Oliver had been able to read the historical accounts of the battle against them that the kingdom of Numa had in its moment.
It was one thing to know that the abysmals are dangerous and another to see historical data of what really happened. Andrew and the others had serious expressions, as the appearance of the abyssals was a matter of discovering whether they would survive or not.
