In the monastery map.
During combat my rogue used his shadow teleport power to teleport to this location and he is now stuck because there's no pathing in the game to get back. Note that ability can only be used once/day, and only in combat. So there's no way to recover other than loading a prior save.
BTW, the same thing could happen with misty step if it runs out while over there and you've none left to cast.