UNREACHED LANDS

 I was inspired by AD&D 1e and Empire of the Petal Throne. When you get Nat20 on to-hit roll against humanoid without a helmet (or against a monster that poorly protected*) you damage roll multiple by 2. * - as an example: Giant Crab with solid chitin counts as well protected. Giant Spider with soft chitin counts as poorly protected. This is about the " Achilles' heel " thing.