I agree with Chronata and Zorya completely, but will just add something else...
I think that the Dark Night of the Soul (the phrase was coined by St John of the Cross, when he wrote a poem about it) is a most important part of one's mystical path... It is a mystical experience all in itself, and as has already been said, leads to something better afterwards.
It is indeed the Tower, where we realize what we previously believed is rubbish, and we are left bereft... But the beauty of this is that it clears the way of rubbish, and allows us to rebuild, better. Hence, the Star card after the Tower, or Temperance after the Hanged Man and Death.
The Dark Night is also a time where you don't move or grow very much - you are, essentially, incubating. And what is it we see on the Art/Temperance card in the Thoth deck? "Visit the interior of the Earth. Through rectification you shall find the hidden stone." (In Latin.) This is an important part of the alchemical work of the soul - the breaking down, decay of the soul (Nigredo - the Black Dragon) as the solve part of the solve et coagula formula. When you're in the depths of despair; when you feel alienated from God and others; when you are in your own personal hell and everything you once knew is falling down around you... When you are stripped of all your pretenses down to the bare bones... It's a test, it's a challenge, it's an opportunity. You get to rebuild, you get to re-evaluate, you get to strive towards light once more.
But most importantly perhaps, when you come out of a Dark Night, you know darkness, you have faced it, and hopefully you have learned to work with it so that it does not overcome you.
Neitzsche had a lot to say about this, though he didn't realize it. He spoke of the Dionysian Man, the tragic pathos, wherein one doesn't curl up into a foetal position and weep at the first signs of a Dark Night, or pain, or suffering. Instead, one says Yes to it all - including the pain and suffering and Dark Night. One descends into the dark, carrying the light of one's Yes-saying into the Abyss so one can see it for what it really is, and see one's own face reflected back... To do otherwise would be saying No to an entire half of life! Instead, he says we should be like Dionysus, who is torn apart but puts himself back together again, better than before. This, he says, is the Uberman.
(As you can guess, these issues are very close to my heart...!)
There are many myths that I personally associate with the Dark Night. Persephone's Descent into the Underworld, and her mother's frantic grieving and searching is one of them.
In Egyptian mythology, the Sun God Ra enters the Underworld every night, and sails through it on a boat. This would be an easy journey, if it wasn't for all the demons of the Underworld (minions of Set) that are there trying to tip the boat over, kill him, etc - anything to stop him rising again in the morning. But every morning he does.
And of course, there's Lord of the Rings, where at the end everything seems hopeless, when you're certain Sauron will win... But one final frantic effort towards the goal and they've won. That's kinda like the Dark Night I think: you feel as though nothing is going to help, but you've just got to keep on pushing and pushing and pushing until you're out of it.
And it's worth it.
A couple of months ago, I was scrying a Lunar Mansion (I've forgotten which one now - sorry!) that is related to the destructive aspect of the Goddess, and during the scrying I had a particularly potent visualization that sums up what I felt when I went through a Dark Night:
The Goddess was holding me by one foot, and I was in the Hanged Man's position, upside down, over a swilring black abyss. I looked up at Her at looked into Her eyes, pleading with Her not to let go. I scrabbled frantically upwards, trying everything to get Her to hold on to me, but she let out a great cackling laugh and let go. I screamed, falling into the Abyss, where I was bereft of light, of contact with the Divine, where demons and unsavoury 'things' pulled and grabbed at me, trying to draw me further in. I stared into a pool of black, inky water, full of crawling things, and saw my face - but my face was ugly, it was distorted, and I saw all my flaws, allmy nasty sides, all my bad habits. I also saw the power I had to cause others hurt and pain, and saw how easily that could be done. And that was probably the worst part of it all - being faced with such stark truth, to be faced with the fact that you're not all nice. But my hand wrenched a lantern from my chest and threw it into the Abyss, and I understood the vision of me in the pool.
And that was where the vision changed to something similar but nevertheless irrelevant to this thread...
I have seen peoples' Dark Nights last for days, weeks, months, or years... There's never really any telling as to how long they'll last, but I think it depends on the person. And sometimes they're not always huge events - I think it is possible to have 'mini' Dark Nights that last maybe an hour or two. But the big ones, they are the ones that tend to bring about the most realizations later on.
Anyway, I have rambled on for far too long... I am eager to hear what others have to say on this subject.
Blessings,
Kiama