This is a special deck of 31 cards that you can use in a multiplayer game with all ten of the Guild decks. There are three ways to play, and the rules for this particular deck will be the same:

A Guild Free-For-All.

Five-headed Giant, with set teams (Azorius, Izzet, Selesnya, Orzhov, and Boros vs Dimir, Golgari, Gruul, Rakdos, and Simic).

Using the same teams, everyone sits in between two enemy players (so the seating would look like Azorius-Dimir-Izzet or something similar, all around the table) and takes their turns individually, like in an FFA, but you are still politically alligned with your teammates.

This deck exists to make sure the game doesn't go on too long, and that it isn't stale or lopsided. No one plays this deck, but taking a page out of Elder Sign, the cards are drawn from it one at a time at the beginning of each round (if the Azorius went first, it would be before the Azorius' untap step). There are three types of cards here:

Lands (Gate or Maze's End), put it onto the table; since there's no player, you can't activate Maze's End's ability, but the game immediately ends if all 10 gates plus Maze's End have been drawn from this deck.

Global effects, like the Leylines and "Renounce the Guilds". Taking a note from Conspiracy, the Leylines (NOT Renounce the Guilds) will be voted on by the active players. If it's decided it will be played (or the vote is tied), it goes into effect, and "you" will mean "a player", so that everyone equally benefits and suffers from them. The Leylines can be targeted for destruction by spells that destroy or exile enchantments.

Non-Fuse dual spells. Until the card is drawn from this deck, the two guilds on the card can choose to cast their half of the spell at any point that they could normally cast them (sorceries on their turn, instants at any point during the round that they have priority).

This puts a very unique clock on a 10 person game that can easily get out of hand. The game will likely end before 31 rounds, but just in case there's a stalemate, this will entice people to be more aggressive.