The giant spider who attacks Frodo in J.R.R. Tolkien’s mythology is Shelob.
Shelob lives in a pass above Cirith Ungol on the route toward Mordor. In The Lord of the Rings, Gollum guides Frodo and Sam into her lair, hoping that she will kill them and leave the One Ring behind.
Shelob stings Frodo, but Sam drives her away with the sword Sting and uses the Phial of Galadriel. Sam then briefly takes the Ring after believing Frodo is dead, before learning that Frodo has been captured rather than killed.
Shelob is described as a descendant of Ungoliant, the primordial spider-like being from Tolkien’s earlier mythology. She is therefore related to, but not identical with, Ungoliant.