Unseen and evil spirits.
Demons are powerful and malevolent beings that hurt and corrupt the living.
Demons were originally angels, but they were cast out of Heaven for rebelling against God. They work in the darkness, and act as corrupters and tempters, influencing people by latching on to their darkest deeds. They cannot force people to do things, but rather can only tempt, taunt and persuade them. It is only when a person is far enough gone that a demon can trick this person into accepting them into their life, at which point the demon will possess them. The possession is not constant and is linked to their host's sins and sexual behavior.
Although seemingly without physical form, demons can take on the appearances of various people and use these forms to appear to their hosts. They are also able to interact with the world while remaining physically unseen at will. They can reveal black eyes in whatever form they take to show their true nature. While possessing someone, their host's eyes will sometimes flash milky white. Vampires can apparently sense their presence in a host regardless of whether they show themselves or not.
Demons are sadistic creatures, bestial and violent in their behavior. They long for the death of mankind and to take vengeance against God for throwing them out of Heaven.
Powers and Abilities
- Possession: Demons can possess humans, but only if the human host is willing to let them in.
- Psychokinesis: Demons can use the power of their minds to move objects without touching them, such as causing doors to slam and windows to shatter, or even causing multiple objects to fly through the air. They have demonstrated this power both within and outside of their hosts.
- Pyrokinesis: The power to generate, control and manipulate fire. Magda set an entire field ablaze; a path of fire followed her with each step, killing many men in the process, including Tiago Vega's father.
- Levitation: Demons can cause the person they are possessing to levitate into the air and hover there.
- Inhuman agility: When in a host, demons can cause their host to pull off incredible feats of agility, such as jumping high into the air and climbing on bare walls.
- Clairvoyance: Demons have shown signs of clairvoyance, able to sense and know information about others such as their sins, pasts, and moral choices.
- Enhanced strength: The demon possessing Vanessa Ives was strong enough to break the ropes restraining her, and it took three men to restrain her while she was possessed. 
- Shapeshifting: Demons are able to take the forms of various human beings both in the minds of their chosen victims as well as to the broader public. They can choose to either assume the form of someone close to their victims to psychologically torment them, or can appear as complete strangers to manipulate events in the human world in order to fulfill their own malevolent designs. 
- Invisibility: Both Lucifer and Magda have displayed the ability to become seen or unseen at will, only perceived by other higher beings such as angels. Lucifer frequently taunted Vanessa Ives as an invisible force watching her from the shadows, and Magda went completely unnoticed at the farm where Tiago Vega's father died by her hand and also at the Mexican-American freeway protest in Los Angeles, where her influence proceeded to invoke a riot.
- Homunculus creation: Magda can create homunculi, who presumably act as extensions of herself. She notably created one to manipulate Dr. Peter Craft in her guise as Elsa. This homunculus had the appearance and manner of a small male child, and was able to answer questions independently. It appeared human enough that even a trained physician believed it was human. Magda absorbed the homunculus back into herself when it wasn't needed.
- Hypnotic suggestion: Although she relies mostly on conventional psychological manipulation, Magda is able to impart "suggestions" into the minds of those that she whispers to while invisible, which can influence their next action, often with disastrous results.
- Exorcisms: Demons can be banished by having an exorcism performed correctly. However in the case of something going wrong this can make the process worse.
- Catholic symbols and prayers: Ethan was able to either temporarily or partially exorcise the demon possessing Vanessa through Saint Jude's prayer and a medallion bearing her likeness. It is unknown if all prayers would have this affect; a Saint Jude's is held as to be used only in the most desperate circumstances.