Each of Ravnica’s 10 Guilds is led by a “Guildmaster”; a figure or group of figures who holds the highest authority within said Guild. They call the shots, they issue commands, but more than anything, each Guildmaster exists as a representation of what their Guild stands for.
The Guildmaster is often, but not always, the highest Challenge Rating (CR) within a Guild, and all of them are extremely formidable in combat. What does this mean for players? It means you don’t fight a Guildmaster except as an absolute last resort. Most of these individuals have held their titles for decades, some for centuries, a couple for over 10,000 years. Intense preparation is needed to stand a chance of survival against any of them. Which means if your party does encounter one, your first and only priority is to make sure the social encounter does not turn into a combat encounter.
If you’re an Azorius Paladin and Rakdos, Lord of Riots wants your seat to sleep in or your helmet to take a shit in, you give it. Now is not the time to stand your ground. Rakdos will splatter you without losing a step, and he doesn’t get arrested.
If you are a Gruul Barbarian, do not throw rocks at Isperia, Supreme Judge. Because even Borborygmos isn’t going to risk that level of suicidal charge just to rescue your ass. The Gruul know the difference between “chaotic” and “stupid”.
If you are an Izzet Wizard, do not get snippy with Niv-Mizzet, the Firemind. He is a moody, vain, mercurial, 15,000 year-old dragon and has barbecued Izzet he likes more for less. You are not his equal, you are hoping to be included on his endless list of ongoing projects.
If Jarad vod Savo catches you trying to manipulate him, they will never find your body.
All Guildmasters, regardless of their alignment or nature, deserve a healthy dose of fear & respect. Standing against a Guildmaster basically means standing against their entire Guild. And no matter what Guild you belong to, they will not go to war just because you wanted to stick to your guns. Ravnica doesn’t have peace, only peace-ish. Maintaining that peace-ish means weighing & measuring the pros & cons of any situation. Every Guildmaster understands this. The balance of power has remained such that no Guild has managed a decisive edge over another in 10,000 years. That status quo is the ongoing goal for Ravnica.
While they make terrible enemies, a Guildmaster can also be your greatest ally. If they call for aide and you answer, they remember it. Having them on your side can make any number of seemingly impossible tasks suddenly possible. If it’s public knowledge that a Guildmaster has their eye on you, your world changes.
If you’ve earned the favor of Lazav, the Multifarious, consider any open contracts on you cancelled.
If you are responsible for a major payday to the Ghost Council, expect some care packages delivered on your doorstep.
If Rakdos likes you, you can get away with almost anything.
Some favor is better than others. The Golgari aren’t the type for public displays of affection, but if you’re in good with Jarad vod Savo, the Undercity becomes a slightly less dangerous place. Likewise, the best one can usually hope for within the Orzhov Syndicate is to be clear of debt, which is the rarest gift they offer. Simic favors are really only good for traveling overseas or if you have missing limbs. Regardless, a Guildmaster’s favor is still a powerful advantage to have and a Guildmaster’s ire is a dangerous target to carry.
For DMs, Guildmasters are a great way of encouraging, in fact, almost requiring roleplaying. These are the most powerful figures in all of Ravnica and getting into combat with them almost guarantees TPK. If your players are interacting with a Guildmaster, it means they are deep in that particular Guild’s territory and no backup will come to rescue them. Your players will have to pay a lot of attention to the situation, the conversation, and their next decisions. Meta-gaming won’t solve things, the only information that will help them comes from any interactions they’ve had with this particular Guild. “What do they want”, “how do they operate”, and “why should they spare me”.
Any encounter with a Guildmaster they can walk away from is a GOOD session.