That mysterious long, narrow, deep black sack makes two appearances in the novella. The first time it shows up is right after Ivan's been given some opium in Chapter 9. While in the opium haze he feels as if he's going through a long, narrow, deep black sack. Falling through is slow and painful, and he wants to reach the bottom, but is also afraid of what's there. He can't seem to reach it. And then, suddenly…through the bottom he goes. Ivan wakes up and comes back to his senses. He starts listening to his soul and first considers the possibility that his life has been lived wrongly.
The second time the sack shows up is right at the end, during Ivan's three days of screaming. It's a similar experience, although this time we get many more specifics:
For three whole days, during which time did not exist for him, he struggled in that black sack into which he was being thrust by an invisible, resistless force. He struggled as a man condemned to death struggles in the hands of the executioner, knowing that he cannot save himself. And every moment he felt that despite all his efforts he was drawing nearer and nearer to what terrified him. He felt that his agony was due to his being thrust into that black hole and still more to his not being able to get right into it. He was hindered from getting into it by his conviction that his life had been a good one. That very justification of his life held him fast and prevented his moving forward, and it caused him most torment of all. (12.3)
We know there's a mysterious force, which is thrusting Ivan through the sack. We also know that at the bottom of the sack lies something that terrifies him, to which he's drawing closer and closer. We learn that Ivan's justification of his life is preventing him from going through the sack, and that's what torturing him most. When Ivan falls through the sack this second time he sees the light.
What is the deal with the long black sack? To us it sounds like it represents Ivan's own life – his false life, which is to say his whole life up to that climactic final realization. The sack is long, dark and torturous to go through, like Ivan's life has been. At the bottom is an escape, which Ivan is afraid of even as he desires it. But we would say that the death at the end of the sack isn't really total death, but the death of Ivan's false life. If it were just death, period, there probably wouldn't be light there. But the death of Ivan's false life means a new beginning. It's the beginning of a life for Ivan's soul. To arrive at that beginning, though, Ivan has to stop clinging to his false.
You might also see the black sack as representing Ivan's sickness. It's painful to go through his illness, and the escape is death. Bodily death certainly would put an end to Ivan's illness, and it's obvious why he would want it even as he's afraid of it. But then, as we just asked, how would there be light at the bottom?
Finally, why does Ivan see the sack twice? We can't say for sure. But the first sack does seem to prefigure the second. It's after falling through the first one that Ivan first realizes both that his life was unhappy and that it might have been wrong. That's the point when his life actually starts looking to him like a long, narrow black sack. Falling through the first sack is a step towards the genuine light that Ivan finds at the bottom of the second sack. What's revealed to him after going through the first sack – that his life might have been a mistake – is what he has to accept to fall through the second.