Menace of Marbles

A bag magically filled with 1000 marbles. When
under inspection, it appears to be an empty bag
of holding, but when overturned by someone
who knows it's purpose, all of the marbles will
be dropped in a 20 foot square. That area
becomes hazerdous terrain, with a DC 10 Dex
save. Marbles return to the bag after 24 hours.
Who said mages never have any fun?