Shelob's lair

Shelob's Lair, also known as Torech Ungol, was located in the pass of Cirith Ungol and served as an impassable labyrinth in which to ensnare the enemies of Mordor. It was also home to the spider Shelob.

Description
The lair itself was at the utmost top of a dangerous staircase, and was shrouded in shadow and darkness. The tunnel walls themselves were smooth, as if worn by immeasurable touching. It was wide enough for Frodo Baggins and Samwise Gamgee to walk side by side with their outstretched hands touching the side walls. Inside were the skeletons and rotting corpses of Shelob's past prey; deeper into the cave could be found sticky, innumberable webs in a vast maze of tunnels. One path led to the Tower of Cirith Ungol, which was blocked by a half-door to keep Shelob out. Other paths led to various other places, most of which were only known by Shelob herself. There is also a foul and evil smell in the air, sensed by Frodo and Sam.

History
In this place Shelob had free rein to do whatever she wished, including devouring the unwitting inhabitants of the Tower of Cirith Ungol; this freedom is allotted by the Dark Lord Sauron in return for her services. Sam and Frodo, during the quest to destroy the One Ring, were tricked by Gollum to venture here in order to cross into Mordor, and only through use of the Phial of Galadriel are they able to venture out. Shelob, through one of her many tunnels sneaks out and poisons Frodo to eat at a later stage. However Sam finds him and fights off Shelob with the Phial of Galadriel and Sting managing to wound her in the abdomen. She slinks back into her cave, leaving her fate unknown; though Shelob could have died of her injuries.

Etymology
In Sindarin, ungol is a word for "spider".