Exile on Drochtrèdia
The Nulling Reef
The Nulling Reef, as far as anyone can guess, surrounds the entire continent. It has veins that pulse a strange blue light, and the only passage that the Imperial navy found through it is treacherous and winding.
Through magics unknown, the reef has blocked all recorded attempts by wizards to try and teleport across it. Sometimes the magic just fails, and sometimes the caster vanishes never to appear again. The same restriction doesn't seem to hold once inside the reef's boundaries, but the continent is still unmapped, so nobody knows where to aim their teleportation spells in any case.