Iron
In the Marvel Universe some Nature schools of magic, specifically the Druidic and Faerie schools, have an extreme vulnerability to iron in any mass equal to or greater than dagger size. Pure iron weapons do double their normal damage to these magic wielders. Iron placed against them (or massive amounts of iron in their area reduces all magic related FEAT rolls they attempt by a –2CS.
Iron can also serve as an effective weapon against certain daemons, undead, and extradimensional creatures.