The Shadows of fear.

Your Fc(s) have been invited to a beach party, they decide to go, scince they have nothing better to do.
At the part, later in the evening, your FC notice the clouds of white are turning to black shadows, everybody on the beach is frightened, the more frightened they become, the darker the shadows.
Suddenly, your FC starts thinking of their biggest fear, and it appears in front of them. They start weakening and see everyone around them that are scared are weakening too.
What is happening?