Illidan Stormrage has gained the allegiance of the Naga, former Night Elves who were magically mutated during The Sundering, and became Youtube.com reptilian creatures. The Warden Maiev Shadowsong pursues her former prisoner, Illidan, across the sea, first to the Tomb of Sargeras. However, Illidan gains an artifact known as the 'Eye Of Sargeras' and wipes out some of Maiev's best soldiers. Forced to call for help, she sends a messenger back to the mainland of Kalimdor. She asks the assistance of Malfurion Stormrage and Tyrande Whisperwind, but Maiev holds a grudge against Tyrande for her actions in releasing Illidan. Although they are able to chase Illidan to Lordaeron, while helping the Blood Elven prince Kael'Thas, Tyrande delays the advance on an Undead army that causes her to be swept away downriver: upon their reunion, Maiev lied to Malfurion, claiming Tyrande was killed in order to prevent Illidan's escape. Malfurion and Maiev prevent Illidan from using the artifact called the Eye of Sargeras, defeat his army and condemn him to death. Illidan tries to justify his actions by saying his plan was to use the Eye to destroy the Icecrown Glacier and kill the Lich King, thereby destroying their common enemy in the Undead, but Malfurion accuses Illidan of having indirectly causing Tyrande's death. Kael'thas then informs him Tyrande may have survived; Maiev's treachery comes out, and the brothers Stormrage join forces to save Tyrande. At this point, Kael'Thas supposedly takes a few more days to meet up with the human forces in the city of Dalaran, the exact same city in which they stopped Illidan. Malfurion then pardons Illidan after they save Tyrande, though he does not revoke his exile. Illidan then states that he could not join the Night Elves even if Malfurion did permit it, because his master will be enraged by his failure to use the Eye properly, and will hunt him down. Illidan departs for Outland, followed by Maiev. It seems as though Maiev would chase Illidan to the depths of the world, as she would do so. Malfurion then remarks that she has become 'vengeance itself'.
In the next campaign, the Alliance Campaign, the Humans are not the main focus unlike in previous games. Instead of the full Alliance, only the Blood Elven units are available (after the first mission), with the addition of the Draenei and Naga (who join forces with the Blood Elves throughout the game). Prince Kael'thas is the leader of the Blood Elves, a group of High Elves that survived the invasion of Quel'Thalas by the Scourge. Originally helping the Alliance, the Blood Elves take the help of the Naga to defend Dalaran and are imprisoned by Garithos, a racist Human leader prejudiced against the Blood Elves. In the dungeons, however, they are eventually rescued by Lady Vashj and her Naga. Vashj then helps free the other Blood Elves and takes them all to Outland through the same portal Kel'Thuzad opened to summmon Archimonde in Reign of Chaos. Outland is the remnants of the Orc realm Draenor, and Lady Vashj lead Kael'thas there to meet their master Illidan. After fending off Maiev and gaining the allegiance of the native Draenei, Illidan is able to conquer Outland, storming the stronghold of the ruling Pit Lord, Magtheridon, where he hopes he will be safe from his failure to destroy the Frozen Throne. His master, the demon warlock Kil'jaeden, catches them nonetheless, and Illidan begs for forgiveness, claiming that he was merely gathering more forces to defeat the Undead Scourge. Kil'jaeden gives Illidan one last chance to destroy the Frozen Throne...or suffer his wrath.
In the lands of Lordaeron, now known as the Plaguelands, Arthas returns to take his throne. After purging the capital city of humans once more, a civil war takes place. The Undead forces splinter into three major factions: Arthas and Kel'Thuzad, loyal to the Lich King; the Forsaken, led by the Banshee Queen Sylvanas Windrunner; and the 3 Dreadlords (Balnazzar, Detheroc, and Varimathras), loyal to the Burning Legion, seeking to avenge the Legion's defeat by controlling the Undead. Although it is speculated that Arthas would dominate all factions, he is forced to journey to Northrend, as Illidan's use of the Eye of Sargeras, while failing to kill the Lich King, has cracked his glacial prison and caused his power and that of his followers to weaken. In the end, Sylvanas defeats the Dreadlords, enslaving Varimathras and slaying the others, to become the nominal ruler of the Plaguelands, while Arthas travels to Northrend to defend the Lich King, stealing the treasure of the ancient blue dragon bull, Sapphiron, and turning him into a vicious Frostwyrm and battling the subterranean Nerubian. Arthas has to go underground through the dreaded kingdom of Azjol-Nerub with the assistance of Anub'arak, Ancient-King of the kingdom. They fight their way through the tunnels of the kingdom and spot a massive Forgotten-One which looks like C'Thun, though slightly smaller and weaker. They flee and manage to escape the depths. They rest as Sapphiron flies overhead to ready their troops and battle the forces of Illidan, Vashj and Kael'thas. After a ferocious battle between the Undead army of Arthas, and Illidan's Blood Elf and Naga troops to siege the four obelisks, which allows access to Icecrown glacier, Arthas defeats Illidan in a duel, and ascends to the Frozen Throne. He becomes one with the Lich King by donning the armor of Nerzhul. The new Lich King's further plans will be explored in World of Warcraft: Wrath of the Lich King.