10 magical guilds hold the power in Ravnica. In theory, each is restricted to its own domain by the enchanted document known only as the Guildpact; in reality, there is no shortage of skirmishes and plays for power. 

The Azorius Senate

As the foremost law-making body on Ravnica, the Azorius Senate combines white and blue magic to create complex, magically binding laws and procedures for their members and Ravica at large (though the latter is harder to enforce, depending on the guild's current influence.) Any Azorius guild member will be quick to point out that their guild's commitment to justice and reason is second to none—their detractors, however, would say that this is the only thing the Senate is ever quick to do. The same bureaucracy and examination prized by the Azorius inevitably bogs down the workings of the guild, until one might wonder if justice will be rendered before the victim dies of old age.

Guildmaster: Isperia the Inscrutable, an ancient, enormous sphinx dedicated to order and the rule of law. 

The Orzhov Syndicate

The Orzhov are a guild of contradictions; they control the dominant faith on Ravnica (centered in Orzhova, the Church of Deals) but their presence is just as strong or stronger in Ravnica's financial sector, where the largest banks are Orzhov-owned. It is said that the Orzhov never break a bargain, and they are happy to provide long life, health, prosperity, and even a spiritual tether stronger than death itself—for a price, of course. Masters of black and white magic, an Orzhov contract can bind departed souls as spirits either in service of the guild or as a reward for useful work. Though many in Ravnica are wary of the guild, often promises of eternal life and wealth are too compelling to resist.

Guildmaster: The Obzedat, a council of extremely old, powerful ghosts who have directed the machinations of Orzhov for millennia.

The Selesnya Conclave

The Conclave brings white and green magic to bear in listening to the voice of Mat'Selesnya, a mysterious force they believe to be the voice of nature itself. Members of Selesnya live in small communes called vernadi, centered around a tree and a dryad that connects the settlement to Vitu-Ghazi, an enormous tree that serves as the Selesnya capital. Guild members are called to subsume their individual identities to the will of the Conclave, as well as to evangelize the guild's message to the Gateless population to try and attract individuals of talent and power. Proponents will bring up the guild's message of peace and harmony; detractors point out that same devotion to unity justify sinister techniques like brainwashing, as well as the "quietmen" who silence the voices of dissent.

Guildmaster: Trostani, a being formed of three dryads melded into one who acts as the voice of Mat'Selesnya in Ravnica

The Boros Legion

By far the most openly militaristic of the guilds, in recent years the Boros have taken control of the Wojek League, Ravnica's primary police force. By controlling white and red magic, Boros devotion to order is said to be second to none— it is so strong, in fact, that the guild will cheerfully arrest, break, smash, interrogate, and burn to the ground anything or anyone they believe has or will "disturb the peace." This contradiction is obvious to anyone who is not actually Boros. Guild forces are centered at the military fort Sunhome, but assorted forts, prisons, and precincts that were once nonpartisan police now all have a strong Boros presence. White and red magic are combined by guildmages to rain fiery judgement on wrongdoers who violate precepts as sacred as murder prohibitions and anti-littering ordinances.

Guildmaster: Aurelia, a fiery-winged archangel whose might is exceeded only by her passion for justice

The Simic Combine

As practitioners of blue and green magic, the Simic include Ravnica's premier biomancers, biological researchers, and physicians. Simic medicine is widely acknowledged to be second to none on the planet; however, the guild's other interests might be conservatively described with the phrase "mad scientists." The guild performs many experiments that bend, blend, and outright alter various organisms in an attempt to make Ravnican wildlife better adapted for their urban homes. However, the various predatory oozes, adamantly unkillable monstrosities, and ceaselessly changing and evolving chimeras produced along the way are met with apathy or a certain intellectual glee at upending the natural processes, regardless of the consequences for the populace at large.

Guildmaster: Prime Speaker Zegana, a merfolk of little magic but tremendous intellect who got her start in the Simic labs before eventually replacing the deeply insane former guildmaster

The House Dimir

Officially, the House Dimir does not exist. Often called "the missing guild," Dimir is on its face a small organization of couriers, diplomats, and private investigators whose power is minuscule in comparison to the other guilds. This is, of course, precisely how Dimir likes it; the power of black and blue magic lets them maintain a second, hidden layer of assassins and spies. This aspect of the guild has secrets for currency— agents frequently use sorcery to steal information from the minds of their victims, leaving them none the wiser of the theft. As such, the secrecy of the House works in its favor; even among members, only the guildmaster and their inner circle can reasonably claim to know of the guild's true direction.

Guildmaster: Lazav, a shapeshifting mage whose true face is as unknown as their origins and purpose

The Izzet League

If the Simic are Ravnica's mad scientists, the Izzet are the absentminded professors; of course, this analogy assumes said professors have access to red and blue magic which, when combined, is capable of warping reality based on thought and emotion. The Izzet are primarily inventors, artificers, and researchers (not counting biology— they leave that gooey work to Simic.) Izzet research ranges wildly from the fascinating to the absurd to the downright hazardous, sometimes in the space of a single experiment. Izzet technology is leaps and bounds ahead of the rest of Ravnica; however, its tendency to explode, distort beyond repair, or implode into a timeless void makes their discoveries slow to gain popularity.

Guildmaster: Niv-Mizzet, an ancient, enormously vain dragon whose IQ is described with exponents and whose patience is measured in seconds.  

The Gruul Clans

Gruul is a guild in decline— though of course it is unwise to point this out to any Gruul guild member. Originally founded to protect and preserve Ravnica's wild places against encroaching civilization, the guild thrived on the power and ferocity of nature, using it to fuel powerful spells of red and green magic. However, very little untamed wilderness remains on the face of the city-world, and the task of preserving the species of the planet via relocation or modification has primarily fallen to the Simic. Nonetheless, the distrust of technology and urbanization remains stronger than ever and the force of Gruul's anger makes for a powerful military presence. The guild continues to serve as refuge for those who which to leave the shackles of civilization behind.

Guildmaster: Borborygmos, a large, angry cyclops loosely acknowledged by the Clans to be the largest and angriest

The Golgari Swarm

The Golgari are a controversial guild even among their own members; though few can contest the good they do for all, their methods are distasteful at best and horrific at worst. Guildmages use black and green magic to manipulate life and death itself. Unlike Orzhov and Azorius, however, they care little for spirits willing and unwilling— Golgari necromancy is concerned with the body, resurrecting it to create undead. As such, every race in Ravnica can be found in the guild; of course, relatively few do so in life. Their power structure is ever-shifting and hazardous, but their goal is singular: reclaiming land above and below the surface of the planet to provide resources, sewage treatment, and mass housing and food for the needy of Ravnica (though though it is only "food" in the loosest possible sense of the word.)

Guildmaster: Jarad, an undead elf who guards his power jealously to safeguard against the unpredictable tides of change in his own subordinates

The Cult of Rakdos

It is perhaps predictable that a guild founded and headed by a demon would embrace some unsavory practices. The Cult of Rakdos has a monopoly on the city-world's entertainment sector, primarily because they have zero scruples about the sort of dark delights that fascinate Ravnica's underbelly. Red and black mages thrive on fire and death, and the passions that fuel them lead to a philosophy of nihilistic hedonism. Despite this, the guild is a haven for artists looking to escape and inspire, and tastes of all kinds can be sated in the dark streets of Rakdos territory.

Guildmaster: Rakdos the Defiler, a sadistic, extremely powerful demon delighting in destruction and gore