Planarch Effect is a campaign setting I have dreamed of for some time. Drawing together Planescape with Mass Effect, along with a host of other influences, it has lately become the default world for my thoughts and feelings around Dungeon World.
The Great Wheel is the whirling mass of the many planes, consisting of worlds beyond counting, each suspended within its own crystalline sphere. Every plane has its own character - from hyperboreal ice planes, locked in the eternal grasp of an endless winter, to the bizarre spinning and colliding cubes of Acheron, home-plane of the Urukai.
Crystalline spheres are all that protect a plane from the roiling dream-currents of the Astral Sea, the void between worlds, where words can wound and thoughts can kill. Traveling between planes unguided is a dangerous art, practiced only by a few foolish mages known as planarchs who pilot their spelljammers between the worlds. Most choose instead to travel on more limited gatejammers, which utilise the ancient dragon-gate network that links the many planes together in an immense web of worlds.
The nexus of the dragon-gate network is the Spire, an impossibly tall tower-city perched on the edge of the whirling elemental vortex known as the Mouth of Pain. As the centre of the dragon-gate network, it serves as a gathering place and seat of government for the interplanar civilization known as the Sigillium.
Established in the aftermath of the Draconic Crusade, the Sigil Council is, in theory, the ruling body of much the Great Wheel. The Sigil is the universal term for the sign that binds the Sigillium together - three small circles arranged equally around a larger circle. In one symbol, it represents a compact made millenia ago by the three Council Races - the Eldarin, the Myrmidons, and the Urukai - to preserve balance in the universe after the fall of Dragon-kind.
Worlds that accept the Sigil are granted the protection of the Council Races, at the cost of buying into an arcane and ancient series of laws, trade restrictions, and commandments that the Sigil Council has established over the centuries. To enforce these laws and the balance of peace across the Great Wheel, the Sigil Council appoints Wardens of the Sigil, elite agents entrusted with extraordinary authority to take any action necessary to preserve balance in the name of the Sigil. Wardens are generally considered to be above any law or sanction beyond that of the Sigil Council themselves.
Most races within the Sigillium have no representation on the Sigil Council themselves. An arcane system of patronage governs the interrelationship between races within the Sigillium - races that have not developed planejamming arts themselves must serve as clients to one of the Council Races. Humans of the world of Urtha, the first race since the Council Races to have developed planejamming arts without the intervention of others, serve as a challenge to the whole structure of the Sigillium as it stands today.
Locales of Note
The Mechanus Veil is a blanket across the edge of the Sigillium where the dragon-gate network has been disrupted by the actions of modrons, ancient machine-beings of unknown origin that once marched across the stars and were only pushed back by the combined efforts of the Council Races. There are rumours that they have grown in activity
Arrathoom is a desert plane near Urtha, peopled by insectoid nomads known as the Tharkeen. In the distant past, it was a lush and pleasant world, but the Dragon-empire defiled it with horrific magical experiments. Now, it is a priceless source of the magical residue known as irradium, an addictive substance which can empower its users with incredible supernatural powers.
The Urukai home plane of Acheron is not a single world - within its sphere, a thousand thousand spinning cubes of metal and iron clash endlessly in the eternal dusk. Within Acheron itself, the Sigil is partially broken - even as they present a united front to the rest of the Sigillium, the five Great Hordes of the Urukai war amongst themselves for honor and a chance to win glory in the next life.
Resting within a lake of fire on the hell-plane of Ignos, the City of Brass is the greatest city in the Great Wheel beyond the reach of the Sigillium. You will not find a more wretched hive of scum and villainy anywhere in the universe. Within its brass walls, slave-traders, pirates, necromancers, raiders and criminals of every description come to meet and trade under the watchful eye of Al-Jabbar, the Efreeti lord of the city.