NOT restricted -- play at will, but be VERY careful not to OP.
Demons (also called infernals) are a humanoid race. They vary widely in appearance, diet, abilities, and habits. Half-demons abound. In fact, it's rare to find a full-blooded demon: they're usually mixed with something else, which negates their weakness to holiness.
Most demons appear human-ish with subtle differences. They can have any colour of hair or eye, with colours atypical of humans being most common. Many demons have strangely-shaped pupils. Often characters can tell something is not quite human about a demon, but not be able to pinpoint precisely what.
Demons cannot use glamours unless they're trained in magic, but they may be able to physically transform their bodies to look more human (turning fangs into teeth, claws into nails, etc.). They're capable of making sounds that humans and elves can't make, such as animalistic roars, snarls, and growls. Their language, Demonic, is physically harmful to most other species and can cause brain bleeds and hemmoraging when overheard.
Demons live a very long time.
It's ridiculously hard to kill demons. They are the cockroaches of Sihiri's dark races, mostly out of spite. Many of them can heal almost instantaneously from even the most grievous of injuries, mending broken bones, stab wounds, and punctures within minutes, and their blood is poisonous if not caustic to other races. Most are faster than humans; some are too fast to track. They have heightened senses and can see in near-dark (though sight's rarely their primary sense). They enjoy mangling corpses, which is convenient because they are often above-average strong. In addition, most have sharp teeth and claws in addition to their opposable thumbs, enabling them to dice you with any sharp thing imaginable. Not that they would mind using dull things. Most demons enjoy pain to the point of ecstasy, especially their own, and will pick a fight just for the rush.
All demons require one specific thing to survive, with that thing differing from individual to individual. Most require death. That means that every week or so they need to be close to a dying thing as it breathes its last breath, preferably with their claws wrapped tight around its throat. Some demons eat urbane things like colours or happiness or toast, making them perfect snacks for stronger demons. The more richly a demon eats the stronger it gets. It also gets stronger with age.
Few demons understand the concept of friendship. Most don't even understand acquaintances. There is an inferred obligation to kill anything weaker than oneself, so meeting with other demons is dangerous at best, deadly at next-best, eventually deadly at worst. For that reason most demons are solitary. If they establish a territory they will often fight to the death to protect it.
Demons keep their weaknesses to themselves. Always. It's considered extremely rude to divulge anyone else's weaknesses, including their name, in public or private. It's also socially forbidden to ask if someone's a pureblooded demon or not (because half-bloods are considered weaker than full-bloods, and because full-bloods are often weak to holiness).
Many pureblooded demons are allergic to holiness. That includes prayer, blessed items, holy water, holy weapons, holy cantrips -- basically anything related to Sihiri's deities. They cannot enter holy places; if they do, they incinerate within a few minutes. Half-bloods aren't usually harmed by holiness, though there have been cases.
It is possible to summon demons, meaning forcibly teleport them to the summoner's location. To do this, an extremely strict set of criteria must be met to the letter. The most well-known method is to get the demon to sign its name in its own blood, then recite the name aloud in the summoner's presence. When the summoner reads the written name while repeating the name aloud exactly as it was spoken, the demon will be forced to appear nearby. There are other methods, all of which lead to raging demons appearing right beside a summoner. And that's a problem for everybody.
It's impossible to starve a full-blood to death: demons who don't eat don't die, they just get crazier. However, it is possible to kill a half-blood through starvation. Many demons are obsessive. They can't stop thinking about things once they start, especially if those things relate to their food of choice, and they eventually feel compelled to act on their thoughts. They also get obsessive over places and people -- they may visit the same places again and again, eventually choosing them as personal territories, and they may run into the person again and again, subconsciously tracking them down through scent and trail. Many demons hate any sense of being owned or compelled and will viciously attack any being they get obsessed with, let alone anyone who tries to control them.
A demon's true name has great power over them. Most go by nicknames or truncated versions of their true names.