Khorvaire is a continent scarred by war. The Five Nations unified by King Galifar the First nearly a millenia ago split into a deadly conflict when the last member of his bloodline died with no clear heir. Instead his five children, each ruling a portion of the kingdom in their own right, declared war on the others. This conflict, which came to be known as the Last War, raged for a hundred years. The end only came with the complete annihilation of the nation of Cyre by unknown means, which frightened the kings and queens of the continent into declaring a truce. Now each of the Five licks its wounds, preparing for the day when they can start the war anew.
And yet, there are things even worse than the spectre of all-out war; demons sealed away in ancient times, extraplanar invaders from the Realm of Madness, and even warforged angry at being created for the Last War and treated as tools. It's a setting rife with dangers...but that's what heroes are for.