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 object, situation, person, or group is known as a phobia. Some examples include agoraphobia, claustrophobia, nihilphobia, 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")