As a leaf falls, so falls the world
Investigating these disturbances, Tyrande Whisperwind and Shandris Feathermoon discovered the combined orc and human forces and naturally (since the orcs had killed Cenarius) came to the conclusion that these outsiders in their forests were responsible for the corruption they sensed. Never being particularly happy to see strangers, Tyrande decided to deal with the issue the typical night elf way -- by killing it. After picking up some furbolgs, Tyrande proceeded to do just that, killing the human paladin Duke Lionheart in the joint orc/human camp.
While Tyrande was practicing her "people skills" by killing people, the undead forces under the command of the Legion attacked the encampment and slaughtered the remaining orc and human forces. Then, despite an orderly retreat into the forests, Tyrande found herself forced to use Elune's power to hide her presence from none other than Archimonde himself. Realizing that the situation was far more dire than a few invading orcs and humans, she made the decision to awaken the druids from their enforced slumber.
Soon, after recovering the Horn of Cenarius, Tyrande awoke Malfurion Stormrage, the head of the druids and her lover over the ages. While there was tension between the two almost immediately (Tyrande pointed out to a Malfurion surprised at her aggressive attitude that she did not have the luxury of sleeping through periods of peril, as he and the druids did), they quickly decided to awaken the rest of the druids to oppose Archimonde. They surmised correctly that the head of the Legion invasion would make his way to the World Tree Nordrassil in order to steal the ancient power of the Well of Eternity that it grew out of, and that if he succeeded, he would effectively become a force like unto a god. In turn, they visited the barrow dens of the Druids of the Talon and awoke them, and then headed to the Barrow Deeps of Hyjal to awaken the Druids of the Claw.
Long before, during the end of the War of the Ancients, Illidan Stormrage (the brother of Malfurion) had taken seven vials of water from the original Well of Eternity (the one destroyed in the Sundering) and used that water to create the well that Nordrassil would eventually grow to conceal. As a result of Illidan's actions, he was imprisoned in the Barrow Deeps, supposedly forever, as his actions were seen as having ensured that the world would once again come under attack by the Legion. Upon seeing his prison in the Barrow Deeps, Tyrande argued that in the face of that invasion realized, Illidan's power could help them defeat the demons.
Malfurion imperiously forbade Tyrande from freeing Illidan, which went over about as well as you might expect, since she immediately told him to get stuffed and led her contingent of forces into Illidan's prison conplex to free him. The way sealed behind her, meaning that Malfurion had no choice but to move on with his original objective. He moved on to awaken the Druids of the Claw. These druids had gone feral, however, over their thousands of years under the ground.
Meanwhile, while Malfurion was working out how to awaken these druids without harming their feral selves, Tyrande was murdering her way through a Keeper of the Grove named Califax in order to free Illidan, once again proving that her first instinct when presented with a difficult situation is to kill something. After Malfurion succeeded in bringing the Druids of the Claw back to themselves, he met up with Tyrande and Illidan, and there was much hugging and happiness to see each other. OK, no, there was not. Instead, Tyrande was mad at Malfurion for not supporting her decision; Malfurion was mad at Tyrande for freeing Illidan and at Illidan for being himself; and Illidan was mad at Malfurion for totally stealing his girlfriend and locking him in a basement for 10,000 years. But Illidan was still willing to help stop the Legion, if only to show Malfurion up in front of Tyrande, and so he ended up leading night elf forces into what is now called Felwood.
In truth I was the one betrayed
Tichondrius had not been idle while Mannoroth had been off getting himself killed. Taking advantage of Cenarius' death, the dreadlord used a powerful artifact known as the Skull of Gul'dan (the skull of the deceased orc warlock Gul'dan, as the name probably suggests to you) to warp and twist the forests near the slopes of Hyjal into dark demonic parodies of their former glory. Illidan learned of this when confronted by a strangely armored human, a race he'd never met personally. After a brief battle that proved neither had the upper hand, he demanded to know what the stranger wanted. Laughing, the death knight Arthas Menethil informed Illidan that he wanted nothing more or less than the defeat of the Burning Legion forces in the area, as their downfall would suit his master, and told him about the Skull of Gul'dan and the artifact's great power.
Tempted by his age-old hunger for such power and determined to prove himself, Illidan found and "consumed" the Skull, transforming from a night elf into a being equally demonic and mortal, seething with fel energies. In his new form, Illidan destroyed the Legion forces in the Felwood and annihilated Tichondrius. (This implies that Illidan had grown even more powerful than one of Archimonde's closest lieutenants.)
However, while Illidan's power was undeniable, both Malfurion and Tyrande were horrified by what he had become. Rather than upstage Malfurion in Tyrande's eyes, he seemingly drove her back into his arms, their previous disagreements forgotten in their shared disbelief at the price Illidan had been willing to pay. Malfurion declared Illidan to be no brother of his and demanded he depart Ashenvale at once, which he did. "So be it ... brother."
The destruction of Cenarius had tipped the scales in favor of the Legion, since it deprived Azeroth of the figure who could have rallied the Ancients to battle them once again, but the mortal races had managed to dispatch two of the most powerful of Archimonde's servants, Mannoroth and Tichondrius. Finally, the Prophet could bring the night elves into the alliance between the orcs and the humans. While none of these forces had any great love for one another, all realized that fighting each other was pointless with the Legion bearing down upon the World Tree and the total destruction of the world entire.
Here these unlikely allies fought a holding action. Victory by force of arms was never the plan. Instead, they fought merely to delay the Legion, destroy as many of its forces as they could, to force Archimonde to make his ascent at the pace their plan demanded. Eventually, the leader of the Legion's forces on Azeroth managed to smash his way through and to the World Tree, which was his ultimate goal. Seeking to draw its power, he ascended it in titanic form only to discover too late that the power he sought to usurp would not allow him to even touch it. Malfurion blew the Horn of Cenarius, awakening nature itself, and the slopes of Hyjal poured forth the ancient wisp guardians who swarmed the eredar lord as he ascended the great tree. The power Archimonde sought to steal forced itself into him, destroying him in a massive explosion that scarred the tree and blasted the sloped of the mountain clean.
With their leader destroyed, only a few demons like Azgalor managed to escape the counterattack of the mortal races, and Hyjal (and the world) was saved at the cost of the pact the night elves had made to prolong their lives eternally. Still long-lived by mortal standards, they would now know aging and death as their long-departed highborne kin had.
This moment of destruction also heralded the final departure of Medivh, the prophetic figure who had forged an alliance between humans, orcs and night elves that had managed to defeat the self-same Legion that had returned to Azeroth due to his actions. While possessed by the essence of Sargeras, Medivh had unleashed orcs who were effectively slaves to the demonic blood of Mannoroth upon the world of Azeroth. Now, Medivh had not only helped ensure the destruction of Mannoroth (and thus freed the orcs from their addiction), but he had made his initial crime contribute to the ultimate defeat of Archimonde and saved the world from destruction. Satisfied that he had expiated his debt to the world, the last Guardian of Tirisfal could finally depart for his final rest.
But not all was so easily settled. The orcs, humans and elves still had enmities and antagonisms to cope with. The Lich King still lurked in Northrend. Lordaeron was still a corpse-choked wasteland. What happened after the fall of the Legion would set the stage for the World of Warcraft to follow. Next week, we will climb the Frozen Throne.
If you want to read more about the figures mentioned in this Know Your Lore:
While you don't need to have played the previous Warcraft games to enjoy World of Warcraft, a little history goes a long way toward making the game a lot more fun. Dig into even more of the lore and history behind the World of Warcraft in WoW Insider's Guide to Warcraft Lore.