Niv-Mizzet is an ancient, vain, temperamental, and super-intelligent dragon. He is the original Parun of the Izzet guild; the original guildmaster who signed the Guildpact 10,000 years ago. He is a dragon wizard and the smartest living thing on the plane of Ravnica. Niv-Mizzet is a perfect representation of the guild he founded, although his temper has cost him more than his share of charbroiled apprentices.
And so Niv-Mizzet has plodded through the last 10,000 years dealing with the feeble minds of the rest of the living, staving off boredom with random acts of folly and flame. It is in his guild that he has found a figment of inspiration. The geniuses and brainiacs of Ravnica flock to him, trying desperately to impress him (or at least not bore him to the point where he burns them). And what does one do who is head, shoulders, wings, and tail above everybody else? One becomes an egomaniac durned quick.
The egomaniacal, capricious, vain, and worthy-of-it all dragon has made sure that his guild and all its trappings all spring forth from him, the very lodestar of omniscient perfection. Niv-Mizzet rarely stoops to concern himself with the day-to-day running of the guild, preferring to craft long-range plans and let his underlings implement the details.
The dragon named the Izzet League as such to ensure that everybody knows who runs the show, and because there just aren't any letter combinations better than those in his own name. The dragon's arrogant display does not stop at words - the Izzet signet is redesigned often, each time becoming closer to a vanity portrait of Niv-Mizzet.
He has no patience for minds that do not inspire him or explode by trying.