After the Hundred Companions and Lews Therin sealed the Dark One back up with the Seven Seals, in the last moment the Dark One struck back and tainted Saidin with a darkness that overwhelmed all those channeling at the Bore at that time, driving them instantly mad, to tear up the world and create chaos, ending the Age of Legends .As those men who weren't with these doomed survivors of the assault on Shayol Ghul were exposed to the Taint by drawing on Saidin, these men slowly were consumed with the same madness that turned Lews Therin Telamon into the Kinslayer.
How the Madness works
Described as an oily slime or film over the One Power, that is forced to be plunged through to reach the source, the Taint comes back with the power and begins to suffuse the body and spirt, building up in the user until finally they are overwhelmed by it. Described by Nynaeve al'Mera as being spidery threads of black stabbing into the brain of the man who is tainted, creating a network of darkness around the brain and creating the confusion and insanity that grips the man who is consumed by the Taint. How this madness effects different people differs according to their personality and their own actions.
List of known madness'
Lews Therin: Hero of the Light and protector of all people, turned to random acts of insanity that kills his family and all who he loves.
Rand al'Thor: Peaceful Shepard, Dragon Reborn and hero of the Light, turned to rage filled and emotionless wreck ready to destroy the world in his anger.
Evin Vincova: Proud Asha'man who turned to the Shadow, convinced by Androl Genhald that people thought his position was comprimised and lashed out at Abors, a fellow follower of the Shadow.
Naeff: Former member of the Andoran Queen's Guards and loyal soldier, turned to believing he can see Myrddraal in shadows.