Medusa, one of the three Gorgons or Graeae, is the only one
of her sisters to have assumed mortal form and inhabited the
dungeon world.

When Perseus was grown up Polydectes sent him to attempt the
conquest of Medusa, a terrible monster who had laid waste the
country. She was once a beautiful maiden whose hair was her
chief glory, but as she dared to vie in beauty with Minerva,
the goddess deprived her of her charms and changed her
beautiful ringlets into hissing serpents. She became a cruel
monster of so frightful an aspect that no living thing could
behold her without being turned into stone. All around the
cavern where she dwelt might be seen the stony figures of men
and animals which had chanced to catch a glimpse of her and
had been petrified with the sight. Perseus, favoured by
Minerva and Mercury, the former of whom lent him her shield
and the latter his winged shoes, approached Medusa while she
slept and taking care not to look directly at her, but guided
by her image reflected in the bright shield which he bore, he
cut off her head and gave it to Minerva, who fixed it in the
middle of her Aegis.
[ Bulfinch's Mythology, by Thomas Bulfinch ]