The creature guarding the pass of Cirith Ungol in J.R.R. Tolkien’s The Lord of the Rings is Shelob.
Shelob is a giant spider who lives in a tunnel above the pass leading toward Mordor. Gollum guides Frodo and Sam toward her lair because he hopes she will kill them, allowing him to recover the Ring. Shelob attacks Frodo, but Sam fights her with the sword Sting and the light from the Phial of Galadriel.
Tolkien describes Shelob as an offspring of Ungoliant, an ancient monstrous spider from the mythology later collected in The Silmarillion. Shelob’s lineage explains why she is far larger and more powerful than ordinary spiders in Middle-earth.
Shelob is not the same character as Ungoliant. Ungoliant belongs mainly to the First Age and is involved in the destruction of the Two Trees of Valinor. The Watcher in the Water is also a different creature: it attacks the Fellowship outside the West-gate of Moria.