Complex sentence: Its last King, Arvedui drowned just before the battle was fought and his son, Aranarth decided that he wouldn't rebuild the kingdom and so became the first Chieftain of the Dúnedain.
Split into simpler sentences: Its last King, Arvedui drowned just before the battle was fought. The son of Arvedui, Aranarth decided that he wouldn't rebuild the kingdom and so became the first Chieftain of the Dúnedain.