The great spider that attacks Frodo in Cirith Ungol in J.R.R. Tolkien’s The Lord of the Rings is Shelob.
Shelob lives in a tunnel near the pass of Cirith Ungol, on the borderlands above Mordor. Tolkien describes her as an ancient and monstrous descendant of Ungoliant, a much older spider-like being from the mythology of the First Age. Shelob is not a servant of Sauron, but she benefits from the darkness and danger surrounding his realm.
Gollum guides Frodo and Sam toward her lair, hoping that Shelob will kill them so he can recover the Ring. Shelob paralyzes Frodo with her venom and begins wrapping him in webbing. Sam, believing Frodo dead, takes the Ring, then returns when he discovers that Frodo is still alive. Using Sting and the Phial of Galadriel, Sam wounds and drives her away.
Shelob is often confused with Ungoliant. Ungoliant is the older mythological ancestor; Shelob is the specific creature encountered in The Lord of the Rings.