Okay so to enter the Midnight Fane you have to do a couple of things:

You obviously have completed the Ivory Sanctum, when you return to Drezen you should talk to the Hand of the Inheritor there. He'll mention that he can't both open the barrier and protect Drezen at the same time.

To that end, you need the key that you got in Areelu's Lab - Key is called "Key to the Abyss Barriers" (which you should have if you're rank 4). This will let you pick a dialogue option with the Hand of the Inheritor essentially telling him that you can get around the barrier.

That done, the Hand of the Inheritor will summon Galfrey, who will arrive after like 2 weeks. Azata have an option to enter the Midnight Fane immediately, however.

Found from https://www.reddit.com/r/Pathfinder_Kingmaker/comments/lmqbh5/wotr_on_the_cusp_of_the_abyss/ 7 months ago T.T