Nightmare

[AllanGronkjaer 76]

Do a have to do secret ending in ng+ as Well for the. Nightmare trophy 

[KazaneHime 1,092]

I just got this, you do need to beat the True Ending to get the Nightmare trophy and it will pop on the clear data screen.

[DesmaBR 3,749]

It pops after finishing True Ending, no need to worry about the secret ending.

 

(But you should watch it anyway)

[DIVIDEDbyZEROOOO 1]

So, if you got the True Ending in your first playthrough, there is a vortex you can interact with upon arriving to the final dungeon, to trigger the hidden quest to get the true ending, even if you did not do all quests in your second playthrough.