The official story
Officially, the city is ruled by the Empress, who is kept alive by the art of Vivimancy that she taught to her family. Her orders are interpreted and carried out by the Council of Pontifices: the heads of the six greatest religions in the city.
The Council are selected at the Proving: an annual gladiatorial tournament to determine which of the faiths have the favour of the gods. Any religion can field a champion; they battle, and the last six standing win their high priest a place on the council.
Of course, nothing’s ever really that simple.
The Empty Throne
The Empress doesn’t give many orders these days. She sits on her throne, kept barely alive, and occasionally musters the strength to sign a declaration. Her family of Vivimancers keep her alive, but the Pontifices do all the actual ruling and make all the big decisions. When things go badly, the Pontifices get all the blame. When things go well, the Empress gets all the credit. This suits the Pontifices just fine: they don’t need to be loved to run the city, they just need to have their hands on the purse strings, and this way they have a beloved figurehead ruler in no danger of actually doing or saying anything inadvisable.
It became obvious long ago that, while the Proving was an excellent way of letting the people think their rulers had the favour of the gods, it was a terrible and unreliable way to select Pontifices. Ruling a city is too important to be left down to chance, or the whims of the arena. The decision of which gladiators will triumph in the Proving is made some weeks in advance, based on the power and influence that the religions have, and only the truly fringe faiths won’t be in on the game plan.
This doesn’t mean the same six religions will always be in charge. Power ebbs and flows, and each of the Pontifices finds themselves caught in long and winding machinations. There have, if the stories are correct, been several occasions on which a deal couldn’t be reached, and the balance of power really did rest on the edge of a gladiator’s sword. Even then, the faiths’ champions knew whom they ought to be allying with.
Where does the power lie?
The Council of Pontifices are responsible for most of the day-to-day administration of the city. The Council appoint people to positions like Chancellor, Captain of the Watch, Minister for Public Safety, and so on: so most of the civic functions ultimately answer to them.
The Guild of Doorkeepers
The Guild maintains and maps the gateways which facilitate travel to far realms, which means they have a stranglehold on trade. They have a working agreement with the Council: the Council don’t try and muscle them out of their tolls, and the Guild don’t knock down Tarsium’s primary trading connections. Confusingly, they Guild has strong links to the Church of Doorkeepers, who are one of the main players on the Council of Six. They try to keep a neat division of interests, but occasionally find themselves at odds.
The Royal Family
The Empress was the first and greatest vivimancer: so good that she’s still alive today. She still has a physical body, she can move and sign pieces of paper, and occasionally weep or gasp in agony. Her descendents were no slouches either, though they never got the knack of binding their souls into a living body the way she did. Instead, their living essence is held in glass jars in the Mausoleum, and they serve as advisors to the Pontifices.
The Family are a treasure-house of senile, peevish knowledge, but don’t have much in the way of political power. Nontheless, the Empress remains a potent figure in the popular mind: sufficiently so that the Pontifices keep her around.
The Empress was a Sand Elf, long ago, before her body was remade and repaired beyond all recognition – and the current Imperial Family are Sand Elves too. The Church of Doorkeepers (and its offshoot, the Forgotten Throne) was the Imperial Faith, though those days are long gone.