Izzet League
The Izzet League keep the water systems running and the heating systems running and every other system of modern convienence and infrastructure running in Ravnica. This, however, is mundane tedium to an Izzet.
This guild is home to the masters of theoretical science, invention, and spellcraft. Their nature is a passion for discovery and knowledge, reckless curiousity, their brilliant minds jumping from one subject to another as whimsy sees fit.
Niv-Mizzet is the leader and parun of the Izzet League. He is the smartest being on the planet, far exceeding mortal comprehension. However, his flippant nature keeps him from simply taking over. Running the world would be tedious after all and Niv-Mizzet hates to be bored. He is also well-known for being the only remaining dragon on the planet; there are numerous dragon-kin, like wyverns or drakes but as far as true dragons go, Niv-Mizzet is it.
The uniforms of the Izzet tend toward the flamboyant, featuring shiny metals and bright fabrics dyed in the colors of their dragon leader’s scales. Niv-Mizzet is quite narcissistic after all. Further proof of the dragon’s vanity can be found in the name of the Guildhall: Nivix, the metal invented by the Firemind: Mizzium, and the name of the guild itself.
The Firemind
The Firemind is an extrasensory link to Niv-Mizzet’s omniscience. It is to be immersed in passionate brilliance. Not all Izzet are in tune with the Firemind though, the honor must be earned by passing a test.
Weirds
“Diametrically opposing energies in self-sealed plasmodermic bubbles make great pets!” —Trivaz, Izzet mage
Weirds are combinations of elementals. That is, they are a combination of fire and water elementals, for example. Weirds were created by Niv-Mizzet himself, supposedly during the Katazar-Razblat Elemental Symposium. The Firemind was bored with the squabbling of the various elementalists (and he hates to be bored) and so he combined opposing elements to create the paradoxical creatures. Then he had his new creations silence the squabbling.
The weirds have come to be the ubiquitous lab assistants to the Izzet.
Source: Izzet Minisite and An In-depth Look At The Izze- Hey, Look At That! by Matt Cavotta