The second invasion refers to the Burning Legion's second attempt to destroy the world of Azeroth through full-scale military action. Strictly speaking, the term refers to the latter half of the conflict usually called the Third War (specifically, the part of the Third War from Archimonde's summoning at Dalaran to his fall at Mount Hyjal), though numerous other events, namely the outbreak of the undead plague, the arrival of the Lich King, and even the First War were lead-ins or preparations for the conflict. The invasion is divided in three parts:
- Path of the Damned, Warcraft III undead campaign, where control of the Undead Scourge is ultimately given over to the Burning Legion.
- The Invasion of Kalimdor - Warcraft III orc campaign, where the orcs and humans ultimately join forces to battle the Burning Legion.
- Eternity's End - Warcraft III night elf campaign, which finally puts a halt to the invasion at the base of the tree of life, Nordrassil.
Archimonde was a lead character in this invasion and attempted to completely destroy Nordrassil. As he climbed the tree, the Horn of Cenarius was sounded, calling thousands of Wisps — ancestral guardians of the forest — to self-destruct simultaneously while attacking Archimonde. The invasion failed with Archimonde's death on Mount Hyjal. It is analogous to the Third War.
In the leadup to the second invasion, demon lords of the Burning Legion were summoned to Azeroth by those who would sow chaos, conflict, and destruction to further their own ascendance. One of the most powerful these forces attempted to summon was Archimonde, also known as "The Defiler". Unfortunately, his great powers made his summoning difficult, but eventually through stealing an artifact of Medivh, the lich Kel'Thuzad found a way. After his return to Azeroth, Archimonde gave leadership of the Scourge to his demon lord lieutenant, Tichondrius, who battled the remaining forces of elves and humans in northern Eastern Kingdoms around the kingdom of Lordaeron and the elven domain of Silvermoon. He personally destroyed Dalaran and then moved his fight to Kalimdor with the ultimate goal of destroying the elven World Tree, Nordrassil. He spent some time helping manipulate the orc forces corrupted by the blood of Mannoroth around Ashenvale forest, but suffered some setbacks such as losing Mannoroth and Tichondrius while heading to Mount Hyjal, location of Nordrassil. The loss of Mannoroth allowed a large portion of the orcs to turn against the Legion.
Archimonde then proceeded with the original objective of the invasion. The mighty demonlord carried out the invasion to attack the World Tree, Nordrassil. By draining the Tree's energies, not only would Archimonde gain untold power, his demonic Legion will also be able to drain the night elves of their source of power and immortality. But Tyrande and Malfurion had allied with Thrall and Jaina Proudmoore, as well as the Shadowtooth dark trolls and the Barkskin furbolgs. Archimonde's ascent up Mount Hyjal was agonizingly slow, being forced to destroy every successive base with the help of Azgalor (Mannoroth's successor as leader of the pit lords), the lich Rage Winterchill, and Anetheron.
The bases, consisting of the humans, orcs and night elves, did slow his ascent - however, his cunning mind and monumental power enabled the Legion to defeat the bases very quickly, going up against Jaina Proudmoore and Thrall, until he reached the night elves' base which he destroyed easily with a wave of his hand. He grew in size as he climbed, determined to destroy the World Tree. He calculated no one in the world at the time had the power to stop him. Yet he miscalculated. It was not someone who could destroy him, but rather many. Archimonde did not think that the gathering wisps were of threat to him or the Legion.
As Archimonde climbed, Malfurion raised the Horn of Cenarius and called to the ancestral guardians. They rose from the forest and amassed around Archimonde. They fell upon him, detonating in a massive explosion that destroyed the demon warlock and incinerated the forests around the World Tree; nature herself would not allow his defilement to occur.