One day when young Rein was hunting some wild beasts in a remote forest, he suddenly came across this dungeon. Rein had already visited a few dungeons but none of them were like this one.
Normally when a person was standing in front of a dungeon entrance, they would be able to immediately guess that this place was a dungeon just by feeling the aura around it. But this dungeon was not giving off any kind of aura even though Rein was standing right in front of it.
The only reason Rein was able to identify the black vortex in front of him as a dungeon entrance was because of his Eye of Truth. Without it he would never have guessed this portal to be a dungeon entrance. That was mostly because it was common sense that when a person encountered a dungeon they would feel it immediately. As that common sense was not working here so only someone like Rein with an overpowering investigation ability would be able to judge this place to be a dungeon.