Complex sentence: The lands along the southern coastline of the eastern continent, collectively called the Lands of the Summer Sea, include Slavers Bay and the ruins of Valyria, the former home of Westeros' Targaryen kings.
Split into simpler sentences: The lands along the southern coastline of Essos are called the Lands of the Summer Sea and include Slavers Bay and the ruins of Valyria. The latter is the former home of Westeros' Targaryen kings.