A Symphony of Frost and Flame

A Symphony of Frost and Flame was the final battle between the Lich King's Scourge led by King Arthas, and Kil'jaeden's agents led by Illidan Stormrage.

Introduction
Moments later, at the Scourge stronghold, King Arthas and Anub'arak make their final plans to defend the Lich King's Throne Chamber.

King Arthas: Time's running out, Anub'arak. We've got to get inside the Throne Chamber before Illidan does.

Anub'arak: The Throne Chamber lies within the frozen peak at the center of the valley. It can only be opened by activating the four enchanted obelisks that surround it. Illidan's forces have already entrenched themselves near two of them. We'll need to drive them back and reactivate the obelisks ourselves.

Meanwhile, Lady Vashj and Prince Kael approach Illidan.

Lady Vashj: We're sorry to interrupt your meditation, Master, but all our forces are in place.

Prince Kael: Once the chamber is open, we will destroy the Frozen Throne as promised.

Illidan Stormrage: Then the hour has come at last. After today, the Scourge will meet its end! Can you hear me out there, Arthas? Its end!

Arthas and Anub'arak are watching the situation from above.

King Arthas: Illidan has mocked the Scourge long enough. It's time we put the fear of death back in him.

Prelude
After forcing their way past the horrors of the dark catacombs of Azjol-Nerub, Arthas and Anub'arak rendezvoused with their army at the peak of Icecrown Glacier, with the intention of claiming the Frozen Throne in the name of Ner'zhul. They were dismayed to find that Illidan's forces had already arrived, were well entrenched around the pinnacle of ice which contained the Lich King, and were poised to claim the four Icecrown Obelisks which when activated would open the prison. Final defeat for Arthas and Ner'zhul seemed all but assured.

The battle
Arthas and his army quickly wiped out a small naga outpost to the south west of the Frozen Throne and used the resources gathered from the ruins of Azjol-Nerub to establish their own base. From there Arthas and Anub'arak launched their attack against Illidan, Prince Kael and Lady Vashj. The battle raged back and forth across the glacier, with both sides taking and re-taking each of the obelisks. Although the undead were outnumbered, the biting cold and Arthas' considerable military prowess eventually turned the tide against Illidan. The blood elves and the naga were routed, and Arthas claimed all four obelisks, allowing him to approach the Frozen Throne. In a last desperate effort to stop him, Illidan confronted the death knight at the entrance; after a short fight Arthas cut the betrayer down, leaving him badly wounded in the snow. The Scourge had triumphed.

Aftermath
Arthas destroyed the Frozen Throne, and merged with Ner'zhul to become the new Lich King. From his Icecrown fortress, the Lich King made plans to rebuild his forces and conquer the rest of the world. Illidan was apparently driven mad by the encounter; he crawled back to Vashj and Kael, who retreated with their remaining forces back to Outland. He brooded in his Black Temple until a brave band of adventurers, assisted by Maiev Shadowsong and Akama, finally killed him, ending his reign over Outland. Later, in Icecrown Citadel, the Lich King was finally slain by another band of adventurers assisted by Tirion Fordring and King Terenas Menethil, ending Arthas's reign and the Scourge once and for all. Bolvar Fordragon eventually became the new Lich King and now commands the "new Scourge" from the Frozen Throne.

Trivia

 * The name is possibly inspired by the fantasy series "A Song of Ice and Fire" written by George R. R. Martin.