The Guildpact is a form of magic that keeps the ten Guild factions of Ravnica from open warfare. After Millenia of open warfare, 10,076 years ago the guilds set aside their conflict to agree to peace, with the Planeswalker Azor writing the law. The ten Paruns agreed to the terms of the contract with Cisarzim of the Gruul being the final signer.
The Signers were Azor I of the Azorius, Razia of the Boros, Cisarzim of the Gruul, Szadek of the Dimir, Niv-Mizzet of the Izzet, Rakdos of the Cult of Rakdos, MatβSelesnya of the Selesnya Conclave, Svgothir of the Golgari, an unknown Simic Parun, and an unknown Orzhov Parun (Possibly a founding member of Obdezat).
Each guild would have a specific job that no other guild could do for the city (which I will go into deeper on the explanation of each guild) and the combined goal of stopping seizures of power over the City of Ravnica. Attempting to take over Ravnica would ensure at least 9 Guilds opposing you and most likely your own Guild as well.
The founding of the Guildpact occurred on the first of Seleszeni, the first month of the Ravnican year {akin to March 1st on Terran Calendars, which is an almost identical calendar to the Ravnican one}.Β
76 years ago the original Guildpact broke when Agrus Kos of the Boros arrest Szadek, the Parun of the Dimir on a technicality. See many wonder why groups that seem bent on destroying the City of Ravnica or are essentially groups of murderers such as the Gruul or Rakdos are even included in the Guildpact. If the Guildpact is about maintaining law and order why would agents of chaos be included?
Well the Guildpact was balanced between these forces of order and disorder. The forces maintaining order are the Boros, Orzhov, Azorius, and Selesnya, while the Gruul, Rakdos, Golgari, and Dimir sowing disorder (with the Simic and Izzet being Nuetral mainly). This balance makes no side of the guildpact have power over the other. When Agrus Kos arrested Szadek for trying to destroy the Guildpact, it broke the Guildpact as the Dimir were doing one of the Guildpact duties.Β
Around the year 10,075 a new Guildpact was formed with Jace Beleren becoming the Living Guildpact. This is the current system of Ravnica and I need to research more into this so expect a part 2Β focusing on the New Guildpact system in a few months, as it is still recent history.