One advantage of the Western ranking system over the one used by places like Korea and Japan is that it can characterise the high-end of power much better. D-, C-, B-, and A-ranks are reasonably objective to define for espers, and a dungeon of a given rank is one that can be reasonably taken down by a small team of two to six espers of that same rank, but S-rank is basically anything that breaks the scale. D- through A-ranks are designed to cover 99.9% of espers and dungeons, so it's not usually that big an issue, but the power distribution has a pretty long tail, and occasionally you get prodigies like Tae-gun—whose matter conjuration powers are unmatched in volume, rate of creation, precision, and duration are unmatched—and horrors like Nightmare—which has been cropping up for decades and has yet to be destroyed.
And you know what's concerning?
What's concerning is that it's been over three months since the last big S-rank dungeon Tae-gun has had to deal with.
Usually Tae-gun has to be sent to an S-rank dungeon abroad about once a month, two would be strange, and three flips over to concerning. It's concerning because, see, when this kind of thing happens, about twice or three times a decade, it's followed by a very short period of time during which an insane number of A- and S-rank dungeons appear all over the world.
And that's due to happen, too.
Nightmare doesn't only appear during those times, but it always appears during those times.
So that's concerning.
Tae-gun is concerned.
He doesn't have anything to do with that concern, though, other than work.