The Mournlands used to be part of the Great Kingdom. During the final battle of the Great Kingdom’s civil war, a magical cataclysm decimated the battlefield and all sides involved. What was left of the area was completely barren, incapable of supporting any plant life. Since then, none of the countries of Ansalon have shown much interest in the Mournlands.
That changed about 20 years ago. A group of explorers from Founder’s Reach went into the Mournlands and found a vast army of orcs, goblinoids, giants, and worse. The orcish warlord, Turrosh Mak, had apparently united the various “evil” races under his banner. Scrying later revealed that Mournlander settlements are incredibly mobile; all structures are simply stationary vehicles. Even the gardens that sustain the army can be moved.
Since then, the Mournlanders have have been a constant thread to the people of Ansalon. They often send raiding parties into nearby nations, looting caravans and taking prisoners, especially near Founder’s Reach. Every large attack force sent into the Mournlands has been defeated in some sort of terrible ambush, though small parties have been more successful.