This article covers a topic that has not been fully explored on the show and thus contains some information taken from the books, in addition to information from the TV series.
Demons are malevolent, monstrous creatures that travel between worlds, disrupting and destroying life around them. They can also be used as weapons or servants by powerful people who could be capable of controlling them.
Demons come in a large variety of species and forms with almost seemingly infinite numbers.
Demons are easily recognizable as they almost always take on monstrous appearances on Earth, unless they are shapeshifters. Their blood, referred to as ichor, is infused with demon energies and is black and viscous. While ichor is not generally dangerous, it is somewhat toxic and poisonous, and some are acidic and can burn what it touches.
To human eyes, their disappearance can take many forms, depending on the type of demon. Some explode or burst into dust, some fade from view, and some crumple into themselves. Some leave behind ashes or smoke, depending on the manner of its death.
The most immediate weakness of demons is their vulnerability to angelic power. However, like the Downworlders they brought into being, they cannot easily, or permanently, be harmed by normal earthly weapons, so most Shadowhunter weapons are marked with runes to strengthen them against demons.
Possession of the Mortal Cup can grant control over demons.
"Greater Demons" are demons who are more powerful and have distinctive identities.
- In an alternate dimension, the rift between the world of demons and humans has been sealed, blocking demons entry to their world.