Most "civilized" Dark Kind that walk the world today look like "people". They might have odd colored eyes or eyes of all one color, but they could look like anyone they want to.
And any thing.
People, Animals, Abominations…
Most Dark Kind have learned the magical skill to take any animal body parts from any source (living and recent dead) and merge them into their form. While it is most efficient to directly take and use a body part, given enough raw material and time they can create any material form. Those without that skill can have others create a husk (material form) for them.
Without a husk a Dark Kind is simply a small cloud of shadow. It is a pathetic form as they can do nothing without someone "giving" them some organic form.
The Husk provides an anchor for the Dark Kin. While in a husk they are free from being blocked (salt lines or magic wards), repelled (by divine powers), summoned, or even dismissed (killed permanently).
The Husked Dark Kin does not require food or rest, taking magical energy from the Dark Kin itself. However to conserve personal energy, they often shape a husk that can eat and will rest. The strength/ speed/ stamina of the husk is dependent upon the flesh used to make it up.
Other skills they might are some limited mind/ emotion/ illusion magic. They can use these with serious restriction in their raw form. Once they are anchored by a Husk, they can use them like any spell/ power.
The Dark Kind have an entire technology based on DarkGlyphs - a runic kind of magic. Their glyphs do minor transformations to the world. Where humans would use a nail, they draw a binding glyph. Where they might work the metal with alchemy and science to make it stronger, they take raw iron and use glyphs upon it. They would place a glyph and a stone gargoyle decoration would "grow" in its place. In most cases, these are just a "special effect". They take roughly the same time to scribe and enchant as it would take a human with the correct tools and materials to do the same thing. The cost in energies/ material to scribe the glyph are slightly less than the cost of the human process. (Since some of these energies are "free of coin cost", they can be provided cheaply).
Like all Deamons, they are beings from various "places" and realms, rather than full sphere. The Dark Kin are truly deamonic energy forms, similar to fey, but who can feed off energies generated by mortals (souls, emotions, etc) to enhance their personal power. They travel from their realms to spheres/ worlds to gain power. Well, they did before they were trapped here by the Pact.
Dark Kin feed off darker emotions and drives: hate, fear, lust, jealousy, frustration, anger, and so on. They gain magical power through feeding. They still can do this. Mostly they "graze" from the world around them, but at times they will strive to be near primary sources of such feelings. Given certain circumstances, they might even pay a human to allow them to feed off them. This can be cathartic for the Human if they are fed upon deeply.
Note: Divine energies of all stripes are too powerful for them to deal with. It causes them discomfort and pain when exposed to them directly. This is why the Demons and Dark Kind are usually adverse to religions of all stripes and endeavor to minimize their impact upon the world.
The World has had Deamons stride it before, thus there have been Demon Wars in the far past. Demon battles have occurred from time to time as minor portals have opened from time to time. Single demons have been summoned and possessed whole bodies. This was all nothing compared to The Great Demon War.
Not that long ago, The Six Stars aligned themselves perfectly. As was prophesized by the Dark Prophet, the Dark Kin were able to access this world.
They poured through. Each Dark Lord took their own tact. Each army did great damage, took lands, claimed bodies and souls, using tactics that fit their King/ Duke/ Lord. Yet, in the end, it was not enough. They were unable to take the world via guile or force. The Dark Crown was captured and surrendered. Yet with his capture, the very gates that would have been used to send the demons home had to be closed. While the existing demons could not be "sent away", it did prevent any more demons from reaching The World.
The Pact was forged. The Pact bound the Dark Kin to laws of conduct. Now both sides had rules they had to follow. The Dark Kin were given some of the lands they had been holding longest. They became their Holdes. As part of the Pact, they are now bound not to traffic in souls or living bodies, not to wage war and avoid causing harm, and to abide by fair laws. (Part of The Pact includes HeartKind talismans). If The Pact is broken, the Dark Kin must give back all Holdes and things of this realm, reducing them all to immaterial blobs of shadow. Wishing to avoid this underpowered state, they vigerously maintain The Pact policing their own.
Effects on the Campaign
There are Holdes spread around the world. The Holde District is a classic example of how these might work. Some might be more mundane, others might be more wild, depending on the environment. The Dark Kind will mostly stick their Holdes. There will be exceptions.
Demon Hunter is now a profession. They possess knowledge of the Dark Kind and a set of magical runes and skills to deal with such creatures, should they exceed the bounds of the Dark Kin.
Demon Hunters might also have the profession of Demon Warder. These Demon Hunters make sure that Holdes maintain their boundaries.
Some Demon Hunters might be Dark Kind. After all, who better to capture rogue Dark Kind?
Demon War Veteran is an option for characters, if the wars were recent. If this option is taken, the character will gain some experience with Demon Kind and warfare. They will have the ability to learn Demonic Runes (like Demon Hunters) and Dark Kind lore. They will also be prone to having "issues" having been unnerved by Demon battles: PTS, flashbacks, phobias, and, of course, a hatred of all things Dark Kind.
While the world will have its own magic, the Dark Kind can augment any existing magic system. Thus you might have Bargainers, those that work with the Dark Kind to augment their magical might.