Level 2 in Nightmare5th is in a familiar place, but requires an unusual means to uncover. You DO have the tool necessary to enter, though... since you've beaten Level 1. You will find the blue candle within. There's nothing of note in the darkness of Level 1, but you will want to return when you have the Whistle so you can deal with that Digdogger. Trust me on this one.
EDIT
5thNightmare
Finished 5thNightmare, 73 deaths (did better than I did on 3rd.qst, still not as good as I'd like).
First, the last bug that I've discovered: The backtrack from the 'road to Ganon' in Dungeon 9. If you run out of bombs against the Dodongos and try to go back to the other part of the dungeon, instead of a passageway you encounter an empty item room. This effectively traps you if you run out of bombs and forces you to reset the game.
Second, the HYPOCRISY! You yelled at me for having special bosses that take more than 8 hits to beat with the SECOND STRONGEST sword, and yet you have rooms with enemies that take 8 hits each from the STRONGEST sword. One such shutter room requires 64 stabs to clear, whereas my boss battle would have involved 12.
Other than being a little tedious with shutter doors (especially so when Gold Darknuts were involved), this quest is a solid competitor. It is VERY hard, and in some cases unfairly so (Dungeon 5 is a good example of this), but since this is the last 'official' quest and you need to beat 4th to play it, I suppose that is alright.