Second invasion

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 dividen 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.

History
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 or Ancestral Guardians to self-destruct simultaneously while attacking Archimonde. This failed with Archimonde's death on Mount Hyjal. It is analogous to the Third War.

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) Rage Winterchill and Anetheron.

The bases, consiting 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.