The River Wars

In the 3rd Century, the human civilizations along the east banks of the Ruvalk and Falls Rivers, stretching from the southern junction of the two rivers up into the Highlands, was prospering. They were still quite primitive towns, without many connections to each other except a tacit peace agreement. The Drow, banished to wetlands of Palath Nenui by the rest of the Elves in the Medieval Age, had begun to experiment with dark magics. A new and tortured race was created, the Orc. The Orcs were great warriors to the Drow in crusades against the elves to reclaim the homeland of Merethyl, however the Orcs rebelled against the Drow in the 1st century and many fled the land to inhabit the mountains to the south, the Bariloch Range. These Orcs, now known as the Orcfolk of Ushat, and the Drow became allied after many years of hostility. The Orcfolk population grew dramatically and in the beginning of the 3rd Century the clan chieftains decided to invade the Ruvalk River basin. 
The Ruvalk militia of the towns was destroyed and the towns razed in a matter of days and the Ushat horde began to march north into the Highlands. In a flurry of action the Wood Elven kingdom of Imradel marched to head them off along with the Munil, the High Elves of the Upper Highlands and the cities of Human folk from the Highlands. The combat lasted for 5 years and was the first of 4 different "crusades" as they're called. The Drow stepped in on the Orc side during the 2nd Crusade, and the devastating for the Ciyobet Empire (being pushed entirely to it's last standing city, Siñan) and for the 3rd and 4th. However they are all collectively referred to as the River Wars, lasting 88 years during the 3rd Century.