What guards the gate to the underworld in Greek myth?

Horae, in Greco-Roman mythology, any one of the personifications of the seasons and goddesses of natural order; in the Iliad they were the custodians of the gates of Olympus. According to Hesiod, the Horae were the children of Zeus, the king of the gods, and Themis, a Titaness, and their names (Eunomia, Dike, Eirene-i.e., Good Order, Justice, Peace) indicate the extension of their functions…