There has been a great deal of change and evolution of the world of World of Warcraft, and to a certain extent, all the available player races have gone through changes because of the events that have taken place. The original release content had lots of dungeons and quests and things going on, but each one seemed to tell the story of a place rather than the story of a people. Like each place, the stories told there seemed static, as the players grew and moved on, the places all remained the same.
The Burning Crusade, however, began to change all that. Instead of just adding new content with each patch, some aspects of the old content were changed as well, with certain characters and peoples coming to the foreground as major antagonists. Players were no longer merely vague adventurers tasked with saving the world from one giant evil monster or another, their characters had vested interests in bringing about some change in their circumstances.
For no group of player-aligned characters was this as true as it was with the blood elves. From the time The Burning Crusade was released, up to now, when the next chapter of the Warcraft story (Wrath of the Lich King) is starting to unfold, the blood elves are the only player faction whose leader has turned into a major boss in a dungeon (not once but twice!), whose capital city has been deprived of one of its most significant residents (who also ended up turned into a major dungeon boss), and whose culture has undergone a complete turnaround over the course of this expansion's expanding storyline.
The draenei, of course, played a huge role in the story of The Burning Crusade, but in the end, they were mostly just very strong supporting characters. The blood elves were the stars of the show.
The lust for power
When we left our heroes last time, things didn't look too good for them. Their betrayal by the Alliance had left the blood elves in a pretty bad position, where they had no choice but to accept the help of the naga under the command of Illidan Stormrage. The way everything transpired, it seemed as though Illidan might even be a more reasonable and inspiring leader than the members of the Alliance could ever hope to be. The fact that he drew so much on the corrupting power of demonic magic seemed insignificant, as he taught them to control and harness its energies to their own will.
Yet the very act of learning to subdue demonic energy to their own purposes started them on a dark path that could only end in madness and destruction. They came to view all energy, no matter what the source, as a resources to be controlled relentlessly and remorselessly, no matter what the cost. Over time, the need to feed their magic addiction festered like a tumor in their hearts and turned into a wicked lust for more and more power. When Kael'thas saw the magical travelling fortress of the naaru, known as Tempest Keep, he viewed it merely as another sourse of power to be conquered and controlled. He waited till most of the naaru had left the floating citadel and then captured it for himself, in order to use it to drain vast amounts of magical energy out of the world around it -- but in the process, he even managed to subdue and enslave the naaru who remained behind aboard Tempest Keep in order to protect it. Kael'thas had become so corrupted by this point that he no longer viewed the naaru as a sentient being all on its own, but as another source of power to feed their magical thirst -- the energy of the Light which flowed through the naaru.