Who is the Main Villain in Hearthstone's Curse of Naxxramas?
Solution 1:
The guy who pops up is Kel'Thuzad, who is an archlich and boss of the Naxxramas necropolis. The hearthstone addon is based on the dungeon from WoW under the same name:
Kel'Thuzad was the founder of the Cult of the Damned and one of the principal agents of the Lich King responsible for the spreading of the plague undeath across Lordaeron.
You can find additional information about Kel'Thuzad and Naxxramas from the wow wiki entry of Naxxramas.
In hearthstone, you will be able to fight him when the 5th wing (Frostwyrm Lair) unlocks, which also requires you to clear the other 4 wings.
Solution 2:
His name is Kel'Thuzad. He was the last boss in World of Warcraft Naxxramas dungeon as well.