Kael'thas Sunstrider is the former Prince of Quel'thalas, The kingdom of high elves. After the death of his father, The king, Kael'thas came to power. He then led his people to the Outland where he would fall under the influence of Illidan Stormrage and then precede to abandon his people in favor of service to the Burning Legion.
Kael'thas Sunstrider is also a boss in the Burning Crusade instance Tempest Keep: The Eye. He is the final boss in this instance and was previously considered to be one of the hardest bosses in the game, due to the fact that his fight consists of five different phases. Killing Kael'thas used to be required for entrance into the two hardest raid instances in the game, Black Temple and The Battle of Mount Hyjal. He also comes back to life and appears as the last boss in the 5-man instance Magister's Terrace, where he is much weaker than he was in Tempest Keep.
Kael'thas also drops 2 exclusive mounts, the first being a Phoenix (310% speed flying mount) and a White Hawkstrider (100% speed ground mount). He also drops a pet Phoneix which is highly sought after by mini pet collectors.