Deep Darkness is one of the creepiest and saddest segments of the game, merely because of its backstory- this is the empty void that the universe will be if you don't defeat what caused all this in the first place, Erodon. Upon killing Dragonhunter once and for all, Dreon appears if you cleared the game 100%, calmly says hello, and brings you into the future, revealing this. There isn't much to do here but you can find a small easter egg- keep going forward until Dreon asks you if you want to leave. Most people would pick yes, but if you say no, a treasure chest will pop up in front of you. Open it and a little note will come out. Pick it up and it reads-
"I like you. You decided to be different from the others. I hope you'll accept this gift. ~Elora" You'll then recieve 100 coins and a 1-up.
Okami's "Reset (Thank You Version)" plays here. Listen:
The mood I was trying to go for by picking this song for this stage is "you haven't won yet and the fate of the world is still at stake".