Nivix is the guildhall of the Izzet League and, more significantly, the aerie of the guildmaster Niv-Mizzet. Nivix is a massive tower, comparable in size to New Prahv. Labs, test chambers, workshops and the like rise higher and higher through the tower, and every level of the tower houses Izzet alchemists, mages, and sages who are constantly theorizing, experimenting, and tossing miscalculated experiments across rooms.
The very top of the tower is of course the aerie of Niv-Mizzet himself. The Firemind is intensely secretive and, as such, only allows for certain individuals to visit him in his lair, and only at the dragon’s behest. Though none of the experimentations of the tower’s residences seem connected, Niv-Mizzet treats them all as if they were part of one grand experiment.