Thunderspire's Seven-Pillared Hall is governed by the Mages of Saruun, the arcane order who tamed this section of the Labyrinth half a generation ago.
They're clever enough that they've subverted several of the ancient minotaur magics to their own use, including the giant Bronze Warders that they use as guards, and they're powerful enough that both the Drow and Duergar are wary about angering them.
The mages have a secret, though - they're not actually present. Regardless of how many Mages might once have dwelt in the Labyrinth, only two remain. One, Paldemar, has gone rogue, and takes the role of Thunderspire's final villain. The other, Orontor, remains in the Hall, trying to keep things ticking over despite the absence of his peers.
The disappearance of the Mages is never explained. The module offers that Orontor "keeps the fact hidden that he is the only one of the Mages of Saruun currently in residence in the hall". It also seems that the Mages have been gone for some time, as Paldemar went rogue "while the Mages were away", and, as we'll eventually see, his schemes have been in motion for an extended period.
Where did the Mages go? The preamble to the Tower of Mysteries area suggests they "departed Thunderspire to engage in the various arcane studies that often occupy their time", although wherever they've gone it seems that Orontor can't contact them, as he ends up needing to hire adventurers to check up on Paldemar.
Given the central role of the Mages in Thunderspire's setting, it's baffling that we aren't told their whereabouts.
It may have been a smart choice, though. It's important that events in Thunderspire can only be solved by the players. The Mages represent a potential deus ex machina that the developers have wisely taken off-stage. Their uncertain whereabouts sends a clear signal that summoning them back to the mountain is not an option, and that whatever salvation is ultimately achieved will come through the direct action of the PCs.
 Despite there being no current information on the Mages, the backstory gratuitously volunteers names for three of the order's founding members - Hasifir, Niame and Samazar. It's a suspicious abundance of information, given that the three aren't referenced anywhere else in the module. Are these an in-joke - possibly holdovers from one of the developers' previous campaigns?