imaginite set that you can only acquire by defeating Kaos on nightmare difficulty.
So you need to gain access to nightmare difficulty to get 100% completion. The problem is, I started my game on hard-equivalent difficulty and I'm not sure how to access nightmare difficulty on the same file. This was because once I beat the game, I decided to really practice to get ready and try some things while I could still play on other difficulty settings (since I read that, as usual, you can't change the difficulty once you are on nightmare difficulty in a Toys for Bob game), but the problem was that--shortly before I decided I was ready to play on nightmare difficulty--my level stars got completely deleted (and I went from 100% to 0% and the story restarted from the beginning) and I had to earn them all back.
Well I re-earned them all back last night, but now I'm not sure how to access nightmare mode, now that I'm actually trying to play on it. Shortly before my level stars were all wiped out, whenever I spoke to the Brain Ancient, he would sometimes say something that I haven't heard him say recently, which was something like: "...But I can actually prove this claim and take you to a place called nightmare world. Want to go?" and then little arrows on the ground would lead you to the battleship like they would for any special level.
Does the Brain have to say that and I have to board the airship when the arrows are pointing to it before I can access nightmare mode? And if so, what do you have to do to make him say it? Or is there some other way to gain access to nightmare mode that I don't know about?