A Siren is a monster (part of demon) of Greek mythology that spreads discord by singing.
A Siren has the power to spread discord by singing. There is no known being that is immune to this. If a person blocks out her song with earphones or turns off their hearing aid then the song doesn't affect them. The power works by interfering with brain activity, the music makes people turn violent, and start picking fights or attacking things for no reason. A Siren's song seems to particularly affect Vampires, most likely down to their enhanced hearing. Sirens enjoy doing this, and seem to take pleasure from watching people turn violent against another.
Sirens are quick to anger, if they are overshadowed, and are jealous beings, their own jealosy can destroy them. However they can also be destroyed by being bombared with a negative version of there own song. Their song alone sounds beautiful, but contains a ton of supernatural acoustics.
The Siren also can drive sailors to their death. This could happen if a Siren was near an ocean or on a rock in the ocean. Although Serena didn't use this, Sirens can still do this.
In Ancient Greece, the Siren's were destroyed when the Muses beat them in a music competition, and the Siren's own jealousy destroyed them.
In the modern day, a Siren called Serena, planned to spread discord throughout the Whitechapel talent show. She spread her song throughout the school, and opening nights. However Benny, Ethan, Rory and Sarah realised something was wrong with her, and after mistaking her for a Mermaid, they realised she was a Siren, and tried to do the same trick, as the Muses did. At the contest, their song made her jealous, so she stormed the stage, and tried to regrab the attention of the crowd, but Ethan turned a negative recording of the song on her, weakening her to the point where she was destroyed.