Niv Mizzet has found a maze that snakes around all of Ravnica and passes through all guild gates. From what I heard, each guild will have a "maze runner" and they will try through get through the maze. The maze runner that completes the Maze will rule all of Ravnica. But all the guilds will try to stop the maze runners from other guilds.
Here is the first guild runner, who happens to be Gruul.
This card kills against Izzet. Those spellcasters will get a bunch of damage dealt to them every time they cast a spell.
Also, Wizards spoiled the regular art for Maze's End the mythic land.
And of course the Dragon's Maze trailer: