The great spider defeated by Sam in J.R.R. Tolkien’s The Lord of the Rings is Shelob.
Shelob lives in a pass above Cirith Ungol, near the borders of Mordor. She attacks Frodo and Sam while they are trying to reach the land of Sauron, and her sting temporarily incapacitates Frodo.
Sam uses the Phial of Galadriel and Sting against Shelob. He drives her away rather than clearly killing her, and Tolkien leaves her ultimate fate uncertain. This distinction matters because the story does not explicitly confirm that she dies.
Shelob is related in Tolkien’s mythology to Ungoliant, the primordial spider-like being from The Silmarillion. They are not the same character. Shelob is also not one of Sauron’s servants in the ordinary sense; she occupies her lair independently and preys on creatures in the region.