She lived on the Seanchan continent at the time of Luthair Paendrag Mondwin's invasion in FY 992. With Luthair's forces opposed by vast armies of natives led by channelers, Deain offered Luthair the use of a ter'angreal she had created, a device called an a'dam that could control the wearer's ability to channel. Attached to another woman by a leash, it would allow that second person to direct the ability, or bottle it up in the wearer so that she could not use it at all. It could also be used as an awesome disciplinary device if the wearer attempted to rebel against the commands of the controller.
The channeler who wore the a'dam was called a damane. By practice, only those women who had the "spark" or the ability to channel without learning became damane. The woman who held the leash was called a sul'dam.
Luthair agreed to pay an unknown price for the device and the secrets of its manufacture. After this, he had Deain leashed with one of her own devices in the The Towers of Midnight in Imfaral. It is said that her screams shook the Towers themselves. Her eventual fate is unknown, but her "gift" allowed Luthair and his descendents to take control of the entire Seanchan continent within three centuries.
It wasn't until recent times that it was discovered that only women who could learn to channel could be sul'dam, and that being sul'dam subtly and over time, taught them to channel. Thus every sul'dam was potentially the very thing they enslaved with the a'dam. Most sul'dam refuse to believe this even when confronted with irrefutable proof.