Which hundred-eyed giant was lulled to sleep and killed by Hermes?

The hundred-eyed giant lulled to sleep and killed by Hermes was Argus Panoptes.

Argus served Hera as the tireless guardian of Io, a woman Zeus had transformed into a heifer. His many eyes made him an ideal watchman: ancient versions vary on whether he had one hundred eyes or simply an extraordinary number that never all closed at once.

Zeus sent Hermes to free Io. Disguised as a shepherd, Hermes used music, storytelling, and magical speech to lull Argus into sleep. Once all the giant’s eyes were closed, Hermes killed him; some versions say he beheaded Argus, while others describe a thrown stone.

Hera commemorated her faithful servant by placing his eyes on the tail of the peacock. Hermes consequently received the epithet Argeiphontes, usually translated as “Argus-slayer.” Argus Panoptes should not be confused with Geryon, another monstrous figure associated with multiple bodies or heads.