Once Kel'Thuzad was whole again, Arthas led the Scourge south to Dalaran. There the lich would obtain the powerful spellbook of Medivh, and use it to summon Archimonde back into the world. From that point on, Archimonde himself would begin the Legion's final invasion. Not even the wizards of the Kirin Tor could stop Arthas' forces from stealing Medivh's book, and soon Kel'Thuzad had all he needed to perform his spell.
After ten thousand years, the mighty demon Archimonde and his host emerged once again upon the world of Azeroth. Yet Dalaran was not their final destination. Under orders from Kil'jaeden himself, Archimonde and his demons followed the undead Scourge to Kalimdor, bent on destroying Nordrassil, the World Tree.
In the midst of this chaos, a lone, mysterious prophet appeared to lend the mortal races guidance. This prophet proved to be none other than Medivh, the last Guardian, miraculously returned from the Beyond to redeem himself for past sins. Medivh told the Horde and the Alliance of the dangers they faced and urged them to band together.
Jaded by generations of hate, the orcs and humans would have none of it. Medivh was forced to deal with each race separately, using prophecy and trickery to guide them across the sea to the legendary land of Kalimdor. The orcs and humans soon encountered the long-hidden civilization of the Kaldorei.
The orcs, led by Thrall, suffered a series of setbacks on their journey across Kalimdor's Barrens. Though they befriended Cairne Bloodhoof and his mighty tauren warriors, many orcs began to succumb to the demonic bloodlust that had plagued them for years. Thrall's greatest lieutenant, Grom Hellscream, even betrayed the Horde by giving himself over to his baser instincts.
As Hellscream and his loyal Warsong warriors stalked through the forests of Ashenvale, they clashed with the ancient night elf Sentinels. Certain that the orcs had returned to their warlike ways, the demigod Cenarius came forth to drive Hellscream and his orcs back. Yet Hellscream and his orcs, overcome with supernatural hate and rage, managed to kill Cenarius and corrupt the ancient forestlands.
Ultimately, Hellscream redeemed his honor by helping Thrall defeat Mannoroth, the demon lord who first cursed the orcs with his bloodline of hate and rage. With Mannoroth's death, the orcs' blood-curse was finally brought to an end.
While Medivh worked to convince the orcs and humans of the need for an alliance, the night elves fought the Legion in their own secretive ways. Tyrande Whisperwind, the immortal High Priestess of the night elf Sentinels, battled desperately to keep the demons and undead from overrunning the forests of Ashenvale. Tyrande realized that she needed help, so she set out to awaken the night elf druids from their thousand-year slumber.
Calling upon her ancient love, Malfurion Stormrage, Tyrande succeeded in galvanizing her defenses and driving the Legion back. With Malfurion's help, nature herself rose up to vanquish the Legion and its Scourge allies.
While searching for more of the hibernating druids, Malfurion found the ancient barrow prison in which he had chained his brother, Illidan. Convinced that Illidan would aid them against the Legion, Tyrande set him free. Though Illidan did aid them for a time, he eventually fled to pursue his own interests.
The night elves braced themselves and fought the Burning Legion with grim determination. The Legion had never ceased in its desire for the Well of Eternity, long the source of strength for the World Tree and itself the heart of the night elf kingdom. If their planned assault on the Tree was successful, the demons would literally tear the world apart.