Nexus towns refer to the 9 towns (3 Aluvian, 3 Sho, 3 Gharu'ndim) new characters use to be able to start in.
They include...
Holtburg Lytlethorpe Rithwic Yanshi Shoushi Nanto Yaraq Samsur Al-Arqas
A number of years ago, six of these towns were destroyed. Since then, new characters can only start within Holtburg, Yaraq, or Shoushi