The Forsaken are a lot easier to talk about than the Blood Elves or any Elves really because they are different inventions. We do not have 10,000 years of history to go over. We simply have to look at Arthas long trek to catch them all and become king idiot of the Scourge to see what happened about Mesos
Essentially, Kelthuzad placed the Lich zhul plague of undeath inside of the grain in the outlaying areas of Lordaeron. This grain turned many of the inhabitants into undead soldiers directly commanded by the Lich King and began to spread the plague turning the living into the, well, un-living. Arthas, being a good guy at the time, tried to stop it but somehow found Team Rocket the Scourge to be his new ally after he found a Master Ball and caught Mewthuzad Frostmourne.