According to old legends, one of the kings that ruled Amra centuries before the Spellplague was a dragonheir. This king made Gauwervyndhal promise that she or any other dragon would never attack the Sword Lands.
Amra fell when the king was assassinated and the lords fought one another for the throne. Amra's castles and keeps were destroyed during the war.
Even after Amra fell, no dragon attacked the Sword Lands ever since.