What does Milton's reference to "gloomy Dis" mean?

Answer 1

Hey Biggles,
The Paradise Lost...intriguing poems...Greek mythology...

"Herself a fairer flower, by gloomy Dis
Was gathered—which cost Ceres all that pain"

Milton is alluding to the abduction of the Greek goddess Persephone (Proserpina for the Romans). She was Demeter's (Ceres for Romans) daughter.
Persephone was abducted by Hades (Pluto, Dis in Latin), the Greek god of the underworld, while she was picking flowers with some nymphs; thus, Milton's choice of words: herself a flower...was gathered...

If you feel like reading more about the gloomy Greek god of the underworld and his wife click here:
http://en.wikipedia.org/wiki/Hades#Persephone

Answer 2

Dis was one of the Gods of the underworld, otherwise known as Pluto. Gloomy sort of goes with any god of the dead.

Answer 3

"Herself a fairer Flow'r by gloomy Dis". Prosperine was the son of the harvest goddess Ceres and Dis, the god of the underworld, Hades, the place of darkness where the dead went, was her father. Dis fathered her and Dis's name was synonymous with gloom.