Kel'Thuzad

From wow.incgamers.com

Kel'Thuzad was formerly a part of the Kirin Tor of Dalaran, acting as a sorcerer and then as necromancer after deserting the Kirin Tor and becoming a principal agent of the Lich King, Ner'Zhul. He was responsible for causing the great Plague, which caused the inhabitants of several human towns, including Brill, to succumb and turn into the undead. And all of this was done by creating infected grain, which allowed the Plague to spread like wildfire throughout the towns. Eventually though, following this dark deed, Kel'Thuzad was "killed" by Arthas Menethil, Crown Prince of Lordaeron, and never would he cause such chaos again. Or so people thought...

A while later, Kel'Thuzad was resurrected by Arthas, and once more became an agent, serving the Scourge and Arthas, even when he merged with Ner'Zhul to become the new Lich King. The resurrected Kel'Thuzad can now be found in Naxxramas, the floating necropolis previously found above Stratholme, now in Dragonblight, Northrend, and is a boss for the player to face. He is also in charge of Scourge forces from Naxxramas, and is the chief lieutenant of the new Lich King, Arthas Menethil/Ner'Zhul.

Tactics

You can see a video of the entire fight here, or from a healing perspective see Sir Castor's video.