The great ocean that separates most of Ruthaar from Thay. There is however a land bridge in the form of The North. But this is an exceptionally long trip in the cold, and ruled over by the sometimes cruel Northerners or Elves who have little patience with war groups or rash and spontaneous humans. The Rudric also extends down to include the major volcanic island chain of Astrich, along with hundreds of other tiny islands that dot it up and down.
This is a guess, because I (Jake, author) have no idea what it should be, the Rudric is approximately 4,000 miles! across from the out jutting part of Thay to the nearest part of Ruthaar.
As a quick note to the history:
Once the Horrors reached the coasts of Thay, they were temporarily set back. It seemed that many of the original Horrors could not swim. Eventually though, they mutated enough that they made it across the Rudric and the other great waters.