The War of Blood
Once, Tamriel was filled to the brim with Orcs. Also at that time, certain Vampires had discovered how to gain someone’s skills by drinking their blood. Wanting to be the best warriors in Tamriel, the vampires waged war on the Orcs after they refused to hand over blood voluntarily.
The battle, called the War of Blood, lasted a total of seventy years. The Orc population in Tamriel was greatly decreased, many dead or had fled to get away from the war. In the end, the Orcs won the battle, forcing the vampires out of Tamriel, for the time being.
Today, you can still see fortresses built by the Orcs to defend themselves, many taken over by bandits or still inhabited by Orcs.