WARNING: The following post contains spoilers for the novel Arthas: Rise of the Lich King, and the final battle with the Lich King in Icecrown Citadel. They're all pretty much contained on Page 2, and I'll give you another warning later just in case you forget about it.
Most players are, by this point in Wrath of the Lich King, more than familiar with Arthas Menethil and his fall from supposed grace into the arms of the Lich King. Players may be slightly less familiar with Ner'zhul, the orc shaman who originally took on the mantle of the Lich King, but we've got an article for that.
This article on the other hand, isn't so much about Ner'zhul, or Arthas, as it is about the Lich King -- the position that Arthas, and Ner'zhul before him, had foisted upon them. What exactly is the Lich King? What is its purpose, and why does it exist? To answer these questions, we have to go very, very far back into Azeroth's history.
Azeroth was created by a group of creatures known as the Titans, a group of almost god-like giants that roam from world to world for a very distinct purpose -- to create and make order. There are a lot of conflicting stories surrounding the Titans, their origins, and the creation of Azeroth -- however, one story stands out above all others; the story of the corruption of Sargeras, champion of the Titans.