In J.R.R. Tolkien’s legendarium, Shelob is the spider-like creature that attacks Frodo in Cirith Ungol.
Shelob lives in the mountain pass near the Tower of Cirith Ungol, where Gollum leads Frodo and Sam. She is descended from Ungoliant, a primordial spider-like being from the First Age, and is described as a huge, ancient predator. Her lair blocks the route toward Mordor.
Shelob bites Frodo and paralyzes him with her venom. Samwise Gamgee believes Frodo is dead, takes the One Ring briefly, and then returns to rescue him after learning that Orcs have carried Frodo into the tower. Sam wounds Shelob with Sting and drives her away.
Shelob is not the same character as Ungoliant. Ungoliant is the far older ancestor and a major force in the mythology of the Silmarillion. Shelob’s episode occurs late in The Return of the King, shortly before the final approach to Mount Doom.