A Raven's Natural Weapon

 A raven itself has few natural predators, but several animals are after a raven's eggs.

Picture from All About Birds

Ravens are quite vigorous at defending their young and are usually successful at driving off perceived threats from owls, martens and other ravens.  They attack potential predators by flying at them and lunging with their large bills. Humans are occasionally attacked if they get close to a raven nest, though serious injuries aren't likely.