From the Glossary of The Wheel of Time

According to prophecy and legend the Dragon will be born again at mankind's greatest hour of need to save the world. This is not something people look forward to, both because the prophecies say the the Dragon Reborn will bring a new Breaking to the world, and because Lews Therin Kinslayer, the Dragon, is a name to make men shudder, even more than three thousand years after his death.