Argus Panoptes a hundred-eyed giant

Argus Panoptes or Argos was a hundred-eyed giant in Greek mythology. He was a giant, the son of Arestor, whose name "Panoptes" meant "the all-seeing one". He was a servant of Hera; one of the tasks that were given to him was to slay the fearsome monster Echidna, wife of Typhon, which he successfully completed.
Hera rewarded Argos for his service by placing his hundred eyes on the tail of her sacred bird, the peacock.

Guardian of Io
However, his main task, at Hera's request, was to guard Io, a nymph that Zeus was involved with. Zeus, in his efforts to approach Io, told Hermes to disguise himself as a shepherd and make Argus go to sleep. As soon as he fell asleep, Hermes killed him with a stone.