The Clown, an embodiment of fear
Fear is an emotion sensed by most humanoids. It is initiated when an individual perceives a danger, such as a threat of injury or death.
The fear of a specific objects, situations, person, or group is known as a phobia. Some examples include claustrophobia, xenophobia and transporter phobia.
Within Humans, fear leads to the release of adrenaline levels within the body. Pavel Chekov was spared the effects of radiation-induced hyperaccelerated aging when the release of adrenaline from his momentary fear of discovering a dead body on Gamma Hydra IV protected him from the radiation on the planet. (TOS: "The Deadly Years")
Franklin D. Roosevelt once said, "The only thing we have to fear is fear itself." However, Harry Kim found out, this is not true; the psychopathic Clown, an embodiment of the emotion, was very much something to be feared. (VOY: "The Thaw")