Officially called Warcraft III: The Frozen Throne, this is the expansion to the wildly popular Warcraft III.
The game is set several months after Warcraft III, and continues some of the loose ends of the original game. It also paves the way for the conflict within World of Warcraft. Some of the highlights are that it details the transformation of Arthas into the feared Lich King, the awakening of the Naga race, the twisting and conversion of the Blood Elves and the breaking of relations between the Horde and the Alliance.
One of the most key things about Warcraft III: The Frozen Throne is that you get to see the world from Illidan's perspective. While you had this opportunity in Warcraft III, it was a very small one and lasted only a few levels. In Warcraft III: The Frozen Throne, Illidan can be easily considered the main character, contending with Arthas and the scourge. This game also shows the power of Kil'jaeden and explains why Illidan must defeat Arthas.
In the end, Illidan and Arthas Battle each other in a cinematic that can easily be described as one of the most epic moments in the Warcraft lore. Illidan however, loses the battle and is forced to flee back to outland leaving the scourge in control of Northrend. Arthas then proceeds to walk up the frozen throne and take on the armor of the Lich King, thus binding his soul with that of Ner'zhul, and forming The Lich King.