[img=http://media.wizards.com/images/magic/daily/wallpapers/Azorius_Wallpaper_2560x1440.jpg]

[i]Ah, Ravnica. City of Guilds. Truly, a crossway of people, ideas and mana. Like any metropolis, it bustles with people, each leading their own lives, crossing and never crossing others'… the ambition of the masses is so low, truly. Survive, live on, repeat endlessly. And yet, such a city could be a keystone to far greater ambitions. Perhaps, after Alara, this is the right plane to focus my attentions on.[/i]

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Welcome to Ravnica, where the ambitions of the few flow into, alongside and against the ambition of the many. You have always known it to be so, and have always observed from a distance, a step-removed from the anthill. Indeed, your ambitions have always aligned with those of the many -- it is your task to safeguard those ambitions, and make sure an individual ambition does not become too great and threaten the livelihood of everyone else. People speak of the Azorius, and the Dimir, but truly, it is you who have kept balance: you are the Transguild Promenade. An organization not affiliated to any one Guild, dedicated to preserving the chaotic balance of Ravnica. Your life is dangerous, however. There are many who would have you arrested, or beheaded, or worse still. Furthermore, recently, unsettling events have been afoot. You have heard rumors of plots and machinations unlike those before, and an even greater threat, an ambition so overbearing the lesser could end up trampled over. 

[hider= in a more direct form]

We are all individuals who find ourselves in Ravnica and have formed, or joined, depending on how long we've been around, an organization called the Transguild Promenade. It's like a secret Guild, not affiliated to any other, tasked with preserving the balance in Ravnica and ensuring an individual Guild doesn't get too powerful. We're going to have to deal with enemies from other Guilds but also a larger, more dangerous plot set up by Bolas and his pawns. 

[/hider]

[hider=Mana, casting and so forth]

Every creature we have encountered, spell we have learned and artifact we have uncovered is stored in our mind as knowledge. Casting a spell is fairly straightforward. When we wish to summon a creature, we pull it from the aether. If some of you are not familiar with how that works, we basically create a 3D illusion. It's an illusion yes, but it is still capable of ripping your head off. As for mana. What I was thinking is that any being with magic needs to be a) in an area with its mana present and b) the larger the spell, the more time and mana required to cast it. So a druid in a forest is going to be in his home turf, expect him to be able to drop a Worldspine Wurm with (relative) ease. Conversely an illusionist might have more difficulty in a forest. In a large city like Ravnica, there's going to be a flow of all colors at all the time, but this flow is, I reckon, not going to be too overwhelming for any individual color. This means any mage can cast spells at any time in Ravnica, but is going to have difficulty bringing big creatures out. 

[/hider]

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more to be added as I give this further thought. This is also a brainstorming thread! 

I think we're pretty full as far as people go.