Xarabydun started out as a town completely underground, and limited in access to levels 25 and under. Since then Ciandra and the Arcanum have built up on the serface, and the level restriction, and indeed, the portal have been removed from the entrance to the underground portion. (access is available through a long tunnel down) Along with being the center for several quests that relate to the Arcanum, the alchemy vendor has special componants for Phials, somtimes called grenades, and other alchemical goodies.
Getting to Xarabydun
A portal to town can be found in Yaraq. An additional portal can be found in the west branch of Umbral Halls. Portal gems to Xarabydun can be purchased in the mage shops of the original 9 starter towns, as well as the Master Mages.