Sirens are a type of supernatural creature that have been encountered by Sam and Dean Winchester that can take different forms to lure and compel humans to perform drastic acts to prove their love and devotion. These acts are usually destructive or violent and usually result in deaths of one kind or another. Not much is known about them, however, due to the fact that they are creatures rarely seen outside of Greece.
A Siren's true form is that of an ugly, hairless, vaguely-humanoid monstrosity with gaunt, inhuman facial features. Like many creatures in the Supernatural Universe, they are able to take on multiple forms to deceive their prey—humans—usually transforming into a person that matches their victims' desire, such as a lover, friend or anything else that person may long for. In ancient times, they lived on islands and forced sailors to chase them, driving their ships onto the rocks.
By using a sort of venom gland in their mouths, they can infect people with large doses of the hormone oxytocin (a hormone that is produced during childbirth, lactation, and sex; sometimes called the "love hormone") or at the very least, one similar to it. Victims with this much of the hormone in their systems experience an intense sense of euphoria and love for the creature that extends beyond just the physical and/or sexual realms. They truly love the creature to their cores and need and desire whatever the creature says it does for itself, and as acts of love and devotion, the ensnared are willing to carry out drastic, most usually violent and deadly requests simply for the asking. These terrible acts are usually perpetrated against those the victim loved before being infected—proving just how much they love the siren. Once these acts of violence are enacted (or even just as they've begun), the siren "gets bored" and immediately vanishes, leaving their victims emotionally broken to deal with the consequences of their own actions.
Unlike many monsters, Sirens do not technically "feed" on humans. Instead, they feel an intense rush of pleasure when their victims carry out atrocities against other loved ones. Like the people they infect, the intense feelings of euphoria do not last long and they quickly grow bored with their targets and venture off to find others.
- Telepathy - Sirens can read the minds of their targets, learning what it is that person desires and yearns for in others and then giving that to them through illusion. This includes not just physical characteristics, but also mental ones. The siren who wooed Dean was able to offer Dean a person with similar interests in cars, women, life views and provide the illusion of someone he could trust (the last being an issue he was having with Sam at this point in the series), most likely by picking the details needed directly from his brain.
- Shapeshifting - Sirens can alter the perceptions into anyone befitting their target, so they can get close to their victims and infect them with their essence. Because a Siren's true visage can be seen in a mirror, it's possible that this is not genuine shape-shifting, but rather just a form of affecting the perceptions of the humans around them.
- Invulnerability - Sirens are unharmed by conventional means of killing, a bronze dagger coated in the blood of one of its victims is the only thing that can kill a siren.
- Venom Compulsion - Once infected, the target becomes completely and utterly enthralled with the Siren and is willing to do anything to prove their love and devotion. The compulsion is so powerful that they do these things regardless of personal beliefs, desires, morals or of the consequences resulting from said acts. Not even Sam and Dean could keep themselves from killing one another, and they were both fully aware of the Siren's nature and that they were infected.
- Bronze Dagger Coated in Blood of Victim - A Siren can be killed by a bronze dagger coated in the blood of one of its victims.
- The Colt - As it can kill all but five things in creation, it can kill a siren.
- The First Blade - Can kill anything in existence
- Death's Scythe - Can kill anything in existence
- Severe trauma - Dean was able to kill a Siren by beating it to death with a baseball bat wrapped in barbed wire.
- Love - If someone truly loves it then it becomes human.
- Nick Munroe (deceased)
In Greek mythology, the Sirens (Greek singular: Σειρήν, Seirến; Greek plural: Σειρῆνες, Seirễnes) were three dangerous bird-women, portrayed as seductresses who lured nearby sailors with their enchanting music and voices to shipwreck on the rocky coast of their island.
Sirens combined women and birds in various ways. In early Greek art, Sirens were represented as birds with large women's heads, bird feathers and scaly feet. Later, they were represented as female figures with the legs of birds, with or without wings, playing a variety of musical instruments, especially harps. The tenth century Byzantine encyclopedia Suda says that, from their chests up, sirens had the form of sparrows, below they were women, or, alternatively, that they were little birds with women's faces. Birds were chosen because of their beautiful voices. Later Sirens were sometimes depicted as beautiful women, whose bodies, not only their voices, are seductive.