Article: you reach this just before the encounter with baralai.  go back to the junction.  This will open the path to the dark knight dress sphere.

What is a summary?
Know that the dressphere  dark Knight  is very powerful and hard to find out exactly where it is  There are two opportunities to obtain this dress sphere, once in chapter 3 and again in 5. In chapter 3 when you are in the bevelle underground there is a junction with three lifts. Take the right lift up and when your at the top look for a place where you can jump back down to the junction without using a lift (its near the bottom and kind of hard to find) and jump down. In the junction take the middle lift to a small secret area and jump on the platform that is there, this will trigger a switch. Take the left platform to back to the top floor, go all the way to the right and take the right lift back down to the junction. Use the middle lift to go up to a second small secret area and jump on the platform there as well,opening up the second hidden switch. If you miss getting this dress sphere in ch3 you can come back in ch5 to claim it. Find your way into the bevelle underground by using the lifts, the process will be identical except there will be no bosses.