What does Frodo use as a light in the dark caves of Shelob's lair?

Frodo uses the Phial of Galadriel as a light in the dark passages of Shelob’s lair. Galadriel gives him the small crystal vessel when the Fellowship leaves Lothlórien, telling him it may be a light in dark places when other lights go out.

The phial contains water from Galadriel’s fountain holding the light of Eärendil’s star. That light ultimately descends from the Silmarils and the ancient light of the Two Trees of Valinor, giving the object a much deeper history than an ordinary lamp.

In the lair beneath Cirith Ungol, Frodo raises the phial against Shelob, helping drive the giant spider back. Sam later uses its light during the struggle and combines it with Frodo’s sword, Sting. Sting is a weapon, not the light source, and the One Ring offers concealment rather than illumination.