The Doom Keeper is a great magical wall that borders the Kingdom of Mysticus to the north. It apparently divides the world as it extends far into the western wastelands, and out to sea. Further, it reaches high into the air and its faint sickly-green glow can be seen from a great distance. It has existed for much longer than the kingdom, and back through history for as long as anyone can remember.
Precisely why the Doom Keeper exists or who established it is unknown, but stories abound. What the stories have in common is that it holds back a great and ancient evil. A favorite bardic story is of a battle during the divided kingdom period that extended to Arduk, the ancient city against the wall, but that halted until the armies could move south, so as not to fight too close to it.