Seven Kingdoms: Seowyn's Crossing
Falwyth is part of the continent of Keshwan, which lies southeast across the Bright Sea from Summerlund. The humans of Falwyth were initially allies with their neighbors, the dragonborn of the Empire of Azmartheon. Eventually, however, the nobility of Falwyth began to believe that the dragonborn were holding secrets from them. In their folly to gain more power, they entered into pacts with infernal powers, marking themselves and their descendants forever as the race of tieflings.
A faction of peaceful tieflings and humans sailed across the Bright Sea to what is now Summerlund, but the remaining tieflings, treating their subjects as little more than slaves, whipped their people into the terrible Black Fire War with the dragonborn. The war was only ended by a world-shattering event known as The Rending. In the wake of this disaster, proud Azmartheon became a desolate desert, and Falwyth was shattered, falling into the sea. Its remnants exist now only as an archipelago of islands, each ruled by a despotic tielfling lord.