Kael'Thuzad is a member of theLorderon order of Magi in the Kirin Tor , and is a well known and powerful illusion Magi.
A member of the Kirin Tor for most of his life, noone has any idea that he has been corupted by Sauron and now works to destabilize the Kingdom of Lorderon.
History
Early History
Kael'Thuzad was born in Lorderon to parents who were members of the Kirin Tor. Because both his parents were Magi users his birth was a celebrated moment since most of such births resulted in exceptional Magi users.
As he grew into a young man he proved himself quite adept with Magi and at the age of 16 he was inducted into the Kirin Tor.
Kirin Tor
See Also : Kirin Tor
Now a member of the Kirin Tor Kael'Thuzad believed all the anwsers he seeked could finally be anwsered. It was around this time his mother who he loved very much was killed in a potion's accident. The death hit Kael'Thuzad very hard, and it was after this that he began his lengthy pass time of reading Magi books.
It was during this time that he would come upon a book that had been originally found during a Numeron Battle against the original forces of Sauron, and had been locked away due to its percieved evil. At the same time as he found this book the Kirin Tor made a decision to suspend Kael'Thuzad for a period of time until he was willing to return to his true work, and stop his endless studying.
This suspension was the final straw for a man who was already on edge, and so he followed the directions that book appeared to whisper to him.
Going to Mordor
See Also :Sauron
Working for Mordor
"I was cast out by my comrades. Exiled. Forced to wander the burning wastes of Mordor...but I was not alone. Not entirely. The voice, now my only companion, guided me to my destination. Strange, nightmarish creatures awaited me at the entrance. I felt my blood run like lava, as hot as mount doom itself. Inside, I bore witness to horrific acts, demonstrations of power, power that could be mine for the asking. Terrified, I ran...but did not get far. All too soon, my choice was made. Too late did I realize that such power does not come...without a price. Now, the world shall pay a far greater toll, for I have returned. I am Kel'Thuzad. And I am the harbringer of your doom."
-Kael'Thuzad Kael'Thuzad returned to Lorderon and immediatly went about mending fences with everyone that thought he was a failure. In only a week he managed to get himself reinstated in the Kirin Tor. With his positon secured again he went about enacting the will of Sauron.
The Cult of Undeath
Main Article : The Cult of Undeath
His first action in order to spread the influence of Mordor was the founding of an organization that would be formed under the pretents of Magi users in the Kirin Tor who were tired of the strict rejection of dark Magi. Of course in actuality it was going to be an organization that anwsered to noone except Kael'Thuzad and would eventually lead the war against Lorderon. On top of simply recruiting members of the Kirin Tor he also wished to create footsoilders for his growing army and thus began taking his message to the people.
For three years Kael'Thuzad worked, preaching his new religion to the masses, promising them immortality and an ideal society free of all the burdens the existing one forced upon its citizenry. Many of Lordaeron's overworked laborers and disenfranchised masses were taken in by this charismatic individual, and willingly sold themselves into service to the Lich King. Kel'Thuzad also used his fortunes and those of his fellow Cult members to taint and corrupt men in positions of power, bringing them into the Cult through favors and gifts. Eventually, many of Lordaeron's villages were completely under the control of the Cult of Undeath and so, it became time for him to move to the next stage of his plan.
Cursing Arthas Menathil
See Also : Arthas Menathil