Janthir is a sacred island for the White Mantle.
The names of several White Mantle items are in reference to Janthir:
Janthir is a place with inhabitants that were said to possess the gift of true sight. It is possible that Janthir is the name of a Mursaat city, possibily somewhere along the bank of the Ullen River. If so, it is hidden by a magical shadow-like ward (as mentioned by Old Joness, the banks of Ullen are shrouded by something "unseen").
Vabbi's Prince Mehtu the Wise mentions that his library at the Halls of Chokhin contains tablets from a mysterious island that disappears. Considering that the library also contains the NPCs of members of the Mursaat lore intenstive GWO Lore Forums, Mehtu may be referring to the Island of Janthir.