Note: This landmark cannot currently be reached within the game.
Janthir is a sacred island for the White Mantle. It 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").
The names of several White Mantle items are in reference to Janthir: