Can't acess Sun door 3 - nightmare

[Sukita]

I finished the Sun door 3 (Arena fight) on normal and hard but after that theres no nightmare difficulty to select, only normal and hard with a star near them. Is there a specific requirement for unlocking the nightmare difficulty arena besides beating the hard version?

Im at the start of chapter 7 playing on hard if it matters.

[Klima]

thers item in the abbys called Turnament invitation. You need it.

[Sukita]

Thanks. That means I only have to progress more in the story

[Revy]

As mentioned above:
You are getting the Invitation from Nightmare mode from The Abyss at the lowest map just before the 4th Fiend Boss.
Abyss area is unlocked just before the "finale", its not missable but you get what i mean.

Extra info:
It is seriously a huge pain to walk all the way down there (am playing on hard btw), except if by then you got Haze or Cloak and the Heaven's Eye (or eagle) to avoid battles if needed and to know what your going to face. Not sure if you can bypass the previous Fiend battles though. Obviously you will also need some of the strongest items.
Finaly there is only 1 TP at start area. Just before the final Fiend boss there is also the last Door to unlock in the game.