When we get to the S-rank training arc arc at Chapter 58, you can skim read or skip the chapter until chapter 78! I have learned a little too late that the events covered here are very jarring and boring to read because it doesn't progress the plot, which I agree it does not. 3 Arcs comprise of these 20 chapters, first is the S-rank training itself, where Shisui learns that he has a version of the EMS. Umbra Noctis arc is basically a character development arc for Shisui where he accepts his tragic past and finds peace. Shisui will also secure himself allies, called the Nocturnae, but they wont really play a part until maybe Tartaros arc. I'd recommend reading up on Umbra Noctis arc, because it sets up the original storyline I have in plan for the Ancient Demons in the future, though you can skip it for now probably because it is FAR into the future. And lastly, the final arc is just a reactions and slice of life type of arc of what happens to Fairy Tail with the disappearance of Shisui. To cut it short, Fairy Tail comes in 11th at X790 GMG, they come back, learn that shisui "dies" (labelled KIA by Magic Council), and then they dont spiral, but they dont progress much, if at all. At the end of this arc, Twilight Ogre attacks (but it's Dark Mages infiltrated as Twilight Ogre), and then Fairy Tail returns to save the day. So that's the summary of Chapter 58 to 78, HOPE THIS HELPS NEW READERS!!!