D&Dtools

The corrosive vapors created by this spell rust any metal
they touch. Nonmagical objects made of iron, iron
alloys, or other ferrous metals take 1d4 points of damage
for every round of exposure. This damage ignores hardness.
Metal armor affected by rusting fog loses 1d4 points
of Armor Class for each round of exposure, as well
as taking damage. An item reduced to 0 hit points is
destroyed. Ferrous creatures within the cloud suffer the
same damage as objects.
Magic items can resist with a successful Fortitude
save. They are entitled to a new save for each round of
exposure, but need not save again once they succeed.
Material Component: Rust particles from a formerly
magic metal item.