Complex sentence: Ser Artys Arryn killed the last of the First Men's Mountain Kings and the Arryns became Kings of Mountain and Vale, renaming the land the Vale of Arryn, and their king wearers of the Falcon Crown.
Split into simpler sentences: Ser Artys Arryn slew the last of the Mountain Kings and the Arryns became Kings of Mountain and Vale. They renamed the land the Vale of Arryn, and their kings wore the Falcon Crown.