War of the Shadow
Unless stated otherwise, all information herein is taken from The World of Robert Jordan's "The Wheel of Time", Chapter 4. A similar entry appears in the Wheel of Time Companion confirming the information available in the main story arc.
The War of the Shadow, or War of the Power, was the conflict that signalled the end of the Age of Legends, though they did not realise it at the time. Since the Bore into the Dark One's prison had been drilled, society had been falling apart, in a period known as the Collapse, which lasted approximately one hundred years. At the end of this period, the forces of the Shadow felt secure enough to act to try and free the Dark One. They turned the technology and advanced knowledge of the One Power of that Age to the making of weapons of war, including the Shadowspawn which appeared for the first time, in great numbers. The people turned to the Aes Sedai to defend them. The Aes Sedai, in turn, were led by Lews Therin Telamon, who, over the course of the next ten years, became known as the Dragon.
Progress of the War
During the first year of the War, both sides used balefire with no regard to the consequences, but with the Pattern in danger of being destroyed completely, both sides simply stopped using it. For the first three years of the war, the forces of the Shadow were ascendant, but after that period, the forces of the Light took back much of what was lost over the next four years, leading to a temporary stalemate, which lasted nearly a year, after which the Shadow once more advanced. It was becoming clear that the Shadow would win and that the Light was fighting a losing cause. This was exacerbated by several of the leading generals turning on Lews Therin and joining the Shadow.
Conclusion of the War
See also The Strike at Shayol Ghul
Several plans were created to try and end the War, but only two are known in detail, other than the fact that peace factions emerged who believed in negotiation with the Shadow, forgetting the aphorism "there can be no truce with the Shadow". The first plan was to use the Choedan Kal to create a barrier around Shayol Ghul. This plan, favoured by Latra Posae Decume, seemed the most likely, as she persuaded every Aes Sedai of significant strength to support this and only this, in what was known as the Fateful Concord. It never came to fruition however, as the Access Keys, which had been prepared away from the sa'angreal, came to be in Shadow held land.
The other plan, favoured by Lews Therin, was to create seven seals, using cuendillar discs as focus points, to reseal the Bore. As no female Aes Sedai agreed to this, due to fear that misplaced Seals could free the Dark one outright, the party that performed it was male only, the Hundred Companions. When it became clear that the forces of the Light would not be able to hold onto the Choedan Kal long enough to carry out the alternate plan, Lews Therin decided to go ahead with his. This plan worked, better than expected, sealing not only the Dark One, but the most powerful of the surviving Forsaken who were all present at the Pit of Doom. There was a high price to pay however, as the Dark One's counter stroke tainted saidin, killing many of the Hundred Companions and sending the remainder spiralling into madness.
In the three and a half thousand years since saidin was tainted, no man could safely channel. Any who did would go horribly insane. The release into society of so many insane men, and the gradual insanity of the remaining male Aes Sedai, led to the Breaking, in which the remnants of society that people tried to rebuild at the end of the war was completely ripped apart.