Asmodeus's History
The Triad was the most powerful collective of upper-level demons. Initially appearing to be loyal to the Source of All Evil, they were considered to be the leaders of the Underworld after his vanquishment. The Triad has been responsible for most of the attacks on the Charmed Ones. Given their large behind-the-scenes presence in events, they can be considered the series' primary antagonists.
They first became known to the Charmed Ones in 2000, but were killed by Cole Turner soon after when he turned on them. They then returned five years later as the demons Asmodeus, Baliel and Candor, who were vanquished by the Charmed Ones twice.
Background
The Triad has always been the second highest evil in power when the Source ruled the Underworld. They worked separate from him, but did his bidding when necessary. However, it was hinted that other demons, such as the Brotherhood of the Thorn, never fully trusted them. Like the Source, they had many demons working for them who they send out to kill the Charmed Ones.
The Rise of the Triad
When the Triad became aware of the Charmed Ones, they sent many demons to kill the sisters. However, when all demons failed, the Triad summoned the half-demon Belthazor to kill them. Belthazor used his human half Cole Turner to get close to the sisters. His plan was to seduce Phoebe, but ended up falling in love with her. This love caused Cole to earn the ire of the Triad, and when they tried to kill him, it caused him to turn on and kill the Triad.
The Return of the Triad
Five years later, The Triad reappeared in different bodies and revealed their names as Asmodeus, Baliel and Candor. It was revealed that they were responsible for the kidnapping of Christy Jenkins fifteen years ago in an attempt to harness the Ultimate Power. Under demonic care, Christy was brainwashed to warp her sense of right and wrong, causing her to ultimately believe that the Triad were good and the Charmed Ones were evil. Their goal was to use the Jenkins sisters to kill the Charmed Ones.
When Christy was found by her sister Billie, Christy did as she was told by the Triad and started to gain her sister's trust while starting to manipulate her to turn her against the Halliwells.
Xar
It was later revealed that a demon named Xar was a former member of The Triad. Seeking revenge and not wanting them to rule the Underworld, Xar ordered his demons to expose the Triad's return to the Charmed Ones. However, the Triad discovered his involvement through Christy and vanquished him.
The Noxon Demons
When Carl and Helen Jenkins returned to meet their long lost daughter, Candor began to fear that Christy's humanity and love for her parents would throw her off the path the Triad intended. To prevent this from happening, Candor sent a pair of unvanquishable Noxon Demons to kill the Jenkinses. In a fit of rage, Christy then killed Candor by impaling him with her fist, at which point he congratulated her for passing the final test.
Return as Spirits
It was later revealed that the Triad possessed a power that allowed them to exist in spirit form after their bodies vanquished. In spirit form, the Triad continued to manipulate Christy and Billie through the demon Dumain, a servant who had once posed as an "imaginary friend" to the sisters. Meanwhile, a former servant of the Triad called Nomed started assisting the Charmed Ones and provided them with information on the Triad's plan. As the Ultimate Battle began, both the Ultimate Power and the Charmed Ones summoned the Hollow.
While possessed by The Hollow, the Charmed Ones appeared in Magic School and vanquished the Triad in spirit form before confronting Billie and Christy. Both sets of sisters then faced each other in the Ultimate Battle, leading to the deaths of Phoebe, Paige and Christy.
Final Vanquish
After the battle, both Piper and Billie travelled back in time to save their respective sisters. However, Dumain tried to manipulate Billie into saving the Triad instead, which causes Billie to realize they were being manipulated. After reversing the Ultimate Battle, Billie tried to reason with her sister, but realized that Christy was too far gone.
While Billie turned to the Charmed Ones, Dumain kidnapped Coop and stole his ring so he and Christy could use it to travel back in time and warn The Triad. However, Billie and the Charmed Ones arrived at the same moment and the sisters vanquished the Triad one final time with potions. Piper then vanquished Dumain while Billie tried to reason with her sister. However, when Christy attacked her with a giant fireball, Billie was forced to vanquish her sister in self-defence.
