Not a real expert, but as a player...
The setting is based on the idea that there are different "realms", or "domains", each one with its own gothic story, technology level, culture, and so on, all seperated by mysterious mists. Each one is controlled by a Dark Lord who is the focus of a sad tale, evil, and also extremely powerful. For example, one is controlled by a benevolent ruler who takes care of his land and hunts down his nemesis - a werewolf. Except he is the werewolf, but is not aware of it.
The rules are basically D&D - with wizards, clerics, fighters, and so on. There are also a few other changes and additions (such as the half-vistani race), but the major change is the addition of Horror, Fear, and Sanity checks. These are will saving throws you make, and if you fail Bad Things happen. The checks are called for whenever you see or realize something revulting/horrible/frightening/unnerving. I thought the idea was good when I read it, but it doesn't really work in my experience. For example, my party virtually had a TPK as a result of realizing we had dinner with a darklord... our response was basically "so what?!". I find overriding the player's roleplaying to be a no-no.
Another major change is that Evil acts will "curse" you, and eventually turn you into an NPC.
The setting is intended for "gothic horror" play, with all kinds of horrible revelations, characters losing their sanity, having nightmares, and so on. Everyone is evil, deceptive, and scheming, and you are bound to be suprised for the worst.
I can't say I like the setting. It is not a setting for heroes - they don't get rewarded or respected, really, have no chance of winning against the ever-present powerful evil forces of the world, and are their own worse enemies (going catatonic instead of bravely fighting the Big Bad Evil Guy when they learn he was really the kindly old man, for example). I prefer my heroes more... heroic. But your mileage may vary.
Yair