The tunnel of Niantiel is the ninth stage in a personal quest through the Tunnels of Set, which involves several methods which aim towards consciousness alteration.
Niantiel is about death. It is not about philosophical abstractions on what death might be about. It is about death as a hard unappealable blow, as a shock that you cannot negotiate with. No buts, no discussion, no tricks, no games to deal with it. Just death, herself deaf to any and every word of reason. An apparent limit to the free creation of realities that is shown through Saksaksalim. The only thing that can't be appealed, which just hits you mercilessly so that the only thing you can do is to try find some way to build yourself back after you've been shattered.