Nexus towns refer to the 9 original starter towns (3 Aluvian, 3 Sho, 3 Gharu'ndim) new characters used to be able to select from for their starting point in Dereth.
They are:
A number of years ago, Martine destroyed the outposts at six of these towns. Since then, new characters can only start within Holtburg, Yaraq, or Shoushi, and with the Throne of Destiny expansion pack, Sanamar.