Pandemonium is the great demon city. It is the supposed great nexus of demons at the "center" of the Void.
Since there has been no human witness to have ever visited the city, or at least one to have returned to tell the tale, not much is known about it, except that it is staggeringly larger than human cities, and possibly larger and more populous than Earth.
As detailed in the Shadowhunter's Codex, Shadowhunters came to know of Pandemonium through demons who can speak the human language, like when the demon Thammuz told Will Herondale about it.[1] Research on Pandemonium has been limited, largely because most Nephilim consider that there are enough demons present in our world without going to find still more of them.[2]