。
Many people died because of the storm.
Literal
Storm [by-によって] many [attributive-の] people [subject-が] died.
によって here marks the cause/agent of an event — 'by means of / due to.' Also used for passive agents (~によって発見された 'was discovered by~'), means of action, and variation/dependence (人によって違う 'it depends on the person'). A versatile case particle.