A snake demon (pl. snake demons) is a particularly ambitious type of demon. Like all demons, they were once human, and either sold their soul or performed enough evil in life to be sentenced to Hell. Those souls whose chief sin in life was envy will eventually be warped into snake demons.
Snake demons are scaly reptilian beings, with slitted green eyes and forked tongues. Many also have retractable fangs with their own brand of demonic venom. Some even have small horns. Snake demons can take a fully human form which resembles their form in life, but they can never disguise their eyes (aside from the use of contacts or dark glasses). Like all demons, they can render themselves invisible. Also, they are physically capable of overpowering any human, in either their own or a possessed vessel.
To put it simply, iron burns. Any sort of anti-hellspawn weapon will likely be made of iron (and blessed for good measure). Iron will even burn a demon who is possessing a human - but touching iron to the human while they are possessed burns them too. These burns heal eventually, though - as they are specific to the demonic presence, they can heal even when the rest of the body can't.
Holy water burns a demon like acid, though they heal these burns quickly, and it usually just pisses them off. Most demons won't cross through running holy water if they can help it though. As with iron, holy water also burns a possessed vessel, but the vessel will heal quickly - and holy water burns vanish if the demon is exorcised. Demon hunters often spike their drinks with this, as an easy way to tell if one of their guests is possessed. Humans don't notice it any more than regular water.
Snake demons have mild telepathy, and can read the surface thoughts of those in their vicinity (though some beings, particularly angelic beings, can block their intrusion). They are also skilled illusionists, but the illusion must be in their vicinity and they must actively maintain it. In addition, medium-grade telekinesis (enough to make a book whirl about the room but only rattle the bookshelf) is sometimes displayed by these demons.
There are many ways to get into Hell - and far fewer ways to get OUT. While demons have an easier time of this than your average tortured soul, they still aren't technically supposed to be breaking free, which makes things… difficult. However, if they leave in a spiritual form rather than a corporeal one, it's easier to slip through the cracks - but this means that they don't necessarily have access to a body, which can be annoying. This spiritual form is, essentially, the demon's soul - or what used to be a soul anyway - and appears as a cloud of oily black smoke. Luckily (for them), all demons are capable of possessing human bodies… whether they're already occupied or not. Corpses don't have an opposing soul to deal with, but most demons prefer possessing living bodies (because living bodies aren't decomposed) - they effectively move into the body with its native soul, and overpower that soul, rendering the body's owner incapable of speaking, moving, or in any way controlling the body. While in this state, the demon can rifle through the body's thoughts and memories with some ease, making it very easy for them to impersonate the person whose body they're in, and therefore pass undetected. However, the native soul can resist this invasion. Once out of Hell, all demons can teleport from place to place (usually using crossroads) even between worlds - they cannot, however, use this method to get out of Hell, only to get to and from the corporeal side of the gates.
Possession can be prevented by certain symbols (usually in tattoo form), and can be reversed via an exorcism. However, exorcisms can be dangerous - demons are often not careful with their 'meat suits'. The human body can be damaged beyond its ability to heal while the demon is inside without affecting the demon - they still waltz around in their puppet just fine (in effect, possession keeps the body in stasis - it can't heal, but it also can't die). Once the demon is forced out (or leaves) however, the body is back to being plain ol' human, and all those injuries hit it at once. Depending on the level of injury, if the shock alone doesn't kill the victim, the wounds often will.
There are large sigils called Devil's Traps, generally involving a pentagram inscribed within a circle along with other symbols. Drawn, painted, carved, or inlaid on the ground or ceiling, they are effective traps for most hellspawn. Once they cross into the circle, they can't leave it again - unless the sigil is damaged and a line is broken. Demons who notice the sigil before walking into one usually break it themselves, if they can (if it's inlaid iron, the best they can do is avoid it). Demon hunters typically put these up all over their homes.
An unbroken line of salt placed over a threshold prevents hellspawn from entering - alternately, they cannot cross into a circle made from it. Similarly, hellspawn (particularly hellhounds) cannot abide the plant known as Devil's Shoestring (a common name for members of the genus Viburnum, which comes from their long and flexible roots). The plants are used for protection, placed over windows and doorways to prevent hellspawn from entering. Holy symbols can also be used to repel most hellspawn - however, this is ONLY true if the wielder of the symbol has true faith in it, AND the hellspawn has no legitimate claim. In that case, the hellspawn cannot touch the wielder of the symbol, or directly do them harm (i.e. fling a bookcase at them). Hellspawn are unholy and cannot abide by holiness - however, if the wielder is corrupted, the symbol loses power. A Faustian contract also negates the effect - if you've sold your soul to the Devil, no divine power will protect you.
Society and Habitat
Snake demons are medium-level demons. They are ambitious, but are punished for their envy by eternally being just out of reach of what they want. Typically they work for archdemons, sometimes as go betweens between them and crossroads demons. They are always eager to serve, and just as eager to betray if it will serve their own ends.