Dark Night of the Soul & Five Phases
The Dark Night of the Soul seems to be an experience of detachment from Spirit, a fall from grace, a loss of connection with God.
Dark nights at different phases
And this can, of course, happen at any phase of the overall process. It can happen at phase one (in Tozan and Underhill's five phases), as a glimpse while we are still stably and exclusively identified as a human being. It can, and will, happen throughout phase two, during the practice and preparation phase. And it happens following phase three, after an apparently stable and full-blown awakening. It can probably happen in various ways even after that, in the deepening clarification and integration of phase five.
Depth
The depths of sense of loss, for any of these, probably depends on a combination of clarity of awakening and attachment to this awakening. During phase three, there may be a strong identification as the absolute, as Big Mind, so when it (apparently) falls away, there may be an equally strong sense of loss and devastation.
Reasons
As Underhill says, there may be two reasons for this fall. One is from the exhaustion of our human self (physical/energetic/mental) from embodying the energies that typically come through during the awakening.
Another, and maybe more important, is our tendency to identify with it, to create a new exclusive identity as the absolute. And this identity is as exclusive as our previously exclusive identity as a human self.
When the detachment from the absolute comes after phase three, we gain experiences which allows us to integrate the relative and absolute, our human self and Big Mind, in a way that seems far more ordinary, far more human. It wears off some of the edges from phase three, such as a sense of accomplishment and of it being special.
From beeing the brilliant sun of enlightenment, it becomes the hazy moon.