Farther West Than West
How it began: Millenia ago, an army fiercer than any ever known sailed in from the west. They quickly conquered significant portions of the main continent and the surrounding islands, and would have controlled the known world had it not been for the combined efforts of the armies of the nation-states. After years of battle, the armies of the continent were finally able to destroy the invaders in a battle so fierce and violent, the earth itself was torn asunder. The large sunken caldera/bay in the southwest corner of the continent was created during this battle.
Repercussions: Although the nation-states were able to defeat the invaders, some lasting effects of their occupation remain. The westerners were so formidable in battle largely because they were skilled at bending the powers of demons to their will. This introduced the infernal-warlock class to the continent, as well as the foul taint of demon possession that lead to the tiefling race. Areas that were near particularly heinous battles started to see normal human parents give birth to tiefling children (similar to what happens to locations near nuclear fallout). Although tieflings today are not considered evil themselves, they are a constant reminder of the most gruesome period in the history of the world, and tend to be shunned by society. The art of demonology has persisted to this day in isolated areas, particularly to barbaric/nomadic peoples, that tend to view it as an advantage to survival, and do not worry themselves with moral complications.
After the invasion was quashed, forts and sentry post were erected along the western coast, to defend against any repeat invasion. The repeat invasion never came, and most of the sentry posts have fallen into disrepair or been abandoned entirely in the generations since their construction. Many expeditions have set out to find the homeland of the invaders, but those that return only report endless ocean.