What is Shelob's Lair?

"Drawing a deep breath they passed inside. In a few steps they were in utter and impenetrable dark. Not since the lightless passages of Moria had Frodo or Sam known such darkness, and if possible here it was deeper and denser. There, there were airs moving, and echoes, and a sense of space. Here the air was still, stagnant, heavy, and sound fell dead. They walked as it were in a black vapour wrought of veritable darkness itself that, as it was breathed, brought blindness not only to the eyes but to the mind, so that even the memory of colours and of forms and of any light faded out of thought. Night always had been, and always would be, and night was all."