Personally not a fan of how secret it is.
On the level AFTER the dark/shadow level on floor 4 you will find it. That is to say, the literal final level where you are fighting normal enemies, not an interlude or miniboss/boss level.
There is a portion of the wall where exactly one tile is slightly differently colored. Move into it, continue exploring it. Exhaust all dialogue/all options, you can't get "locked" out of it at any point, so no need to worry.
The ending works exactly the same way as A and B, so it will be apparent when you get it.