Luck was on Aaron's side.
The dungeon he entered was the dungeon that fulfilled the required conditions for him to enter Rank-5. In fact, the power levels of the monsters present are much higher than required.
The minimum level of the monsters was at 1100, and the maximum was at 1300. There were only three levels in total.
The first level was filled with monsters between 1100 and 1200, while the second level monsters were between 1200 and 1300 levels.
Even though that was the case, when he first entered the dungeon, he still pushed through the monsters in the first level because these monsters were undead. Every monster is an undead.
Some were skeletons, some were ghouls, some were spectres, and so on, but there was not even a living monster present here, which made him think that the Boss would most likely be an undead monster.
