The Hollow Knight we killed was tainted, but apparently something that traveled the breadth of the kingdom is perfect?
Herrah is certainly the most out-of-the-way of the Dreamers, and that’s probably why she’s also the only one lacking for a guard. There’s no boss fight for her at all, regardless of the order you take these on. Yes, there is a huge maze filled with the nasty Devouts, but it’s not a boss fight. (The Watcher Knights are another fight I try to avoid if possible, but they’re required for every ending so I really couldn’t just put them off anymore. Uumuu, meanwhile, is a walk in the park.) I’ve definitely learned the maze better, but I still have no idea where I am in it. I just know the way forward.
Interestingly, the only way to get this ending is to obtain it before doing the things that would let you understand it. The point, here, is that the Hollow Knight that we killed was unable to completely seal the light away, so the Dreamers were added to make the seal stronger. But even that was not enough, and you need to open the seal completely to replace the vessel for the seal with yourself. But you only get this lore if you complete the story enough to cause the second ending to occur. Perhaps that was Team Cherry’s way of ensuring most players would do more than the bare minimum.