Here are the basic ideas behind this world.
Game mechanics will be standard D20 D&D with a few changes. Magic will be very rare. Wizardry and Sorcery are mostly skills of the ancient past. There are lingering magical items created by earlier societies, but they will not be publicly distributed. The gods have stopped providing clerics their traditional spiritual support for unknown reasons. All divine magic is ceremonial, and current efforts to generate clerical spell slots requires multiple believers performing hours of rituals. Blood magic is manifested in relics of a past age, where placing permanent points of HP damage will power a given magic item for a period of time. All divination, long-distant communication, and teleportation magic does not work in this world.
Gameplay will begin as an urban adventure in a walled city. The city is constantly surrounded by deadly roving creatures who swiftly slay anyone who goes too far from the walls. The knowledge of other cities exists, but no communication has confirmed if any are still alive for hundreds of years. Food for the city is provided by farming fields outside the walls, but these farming groups require large defense forces to work, and the general production of the entire city is depressed by the sheer number of people required to produce food and resources. The city is self-reliant and can stay that way forever... in a general static way. Extreme effort to maintain this stasis will be a theme for the beginning of the adventure.
I am pondering ideas of a bay to provide small fishing resources. The bay could have a man-made reef at the opening to keep large predators from feeding on the fisherman. I've also go ideas for a large quarry under one side of the city to provide stone and spring access. The roving slayers will be land-based, but high-altitude creatures will also keep the PCs from casually flying out till they reach a certain level. I want the players to feel locked in the city till they reach a certain level. Then things need to ramp up.
The city is ruled by an unseen and apparently passionless force. The main keep has people who come and go from it, but they don't talk about their rulers and people promoted to such level go through extensive loyalty testing before being allowed to do so. Despite this rather dictatorial government, the street life is mostly anarchistic. Guilds thrive and channel tight resources to minor leaders within the community, and black market competition will inspire most of the early adventures our party will face. The presence of the keep ensures a lot of good behavior, but how those petty underlords work out their differences is of little concern to the rulers of the city. As long as death and suffering don't become disruptive to the city as a whole, the stasis is kept.
I have a general theory for why the gods are no longer present, and this event has also locked the sun in place in the sky. This world has stopped turning, and it is only rumors that say it ever truly did turn. Under the starting city, they will discover some ancient civilization remnants that offer wizardly resources (so the player can start slinging spells) and a few interesting blood relics (so they have magic items). After they are strong enough to move out of the city, they will find that the divine dead zones are specifically targeting cities, and wilderness existence restores divine spell levels. Once they start to learn these secrets, they will be prodded into returning to their starting city to liberate it from the oppressive group that controls it with behind-the-scenes blood sacrifice. But once they liberate the city, they will learn other terrible secrets that force them to liberate other cities lest giants destroy all of mankind.
The eventual goal of the game is to liberate the world, start the planet spinning on it's axis again, reawaken connection to the gods, defeat this giants, and return home as God-Emperors of the planet. I hate that I've managed to put together a form of medieval SG-1, but the analogy does kind of fit.
1) Has anyone done successful blood magic in D20?
2) Has anyone tried to run low magic D&D worlds and run into significant problems?
3) Have I missed some glaring logical issue here that will make my game truly suck?