Any demon in the underworld can become the source. However they must prove their worth to the various leaders of the underworld and must take part in a ceremony that gathers all the evil in the world and infused it with said demon. The ceremony has to be done by a dark priest and a spell must be read from the Grimoire. This demon then becomes "The Source of all Evil." the leader of the underworld and the leader for the destruction of good.
The First Source
The main background through out the entire the Source as responsible for all organized attacks against the Charmed Ones
. His first on screen appearance was in the episodes "All Hell Breaks Loose." In which the Charmed Ones magic was exposed on national television. He agreed to have one of his minions to reset time in order to save magic. But he had a price. He required one sister Phoebe
to stay behind. Knowing that Piper
would be killed and Prue would soon follow. He then planed to kill Phoebe. Thus ending the Charmed line. After the death of Prue
he went after the half sister Paige
. Because of an ancient law that allowed a young witch to choose either good or evil he possessed her boyfriend in an attempt to lure her to the dark side. However Paige saw through his attempt and choose the side of good. However before he left he vowed to break the Charm circle again.
The death and rising of the second Source
During one of his many "final" battles with the Charmed ones the Sourced revealed the Hollow. An ancient energy that absorbs all magic. During the initial attack he sent a demon infused with the Hollow to steal the powers of the Charmed Ones leaving them defenseless. However he only managed to capture the power of the
oldest sister Piper. But later captured the powers of Paige. When they tried to summon their powers back they inadvertently summoned the Source. A battle ensued and just as he was about to destroy them Cole Turner
infused with the Hollow absorbed his powers and the Charmed Ones defeated him. It was later revealed that the Source didn't in fact die his essence was transported to Cole. In the beginning Cole's love for Phoebe was enough to keep the Source at bay. But over time the Source gained more and more control. He began to reorganize the underworld. His final plane for ultimate power was to marry Phoebe and produce an heir. His plan worked he married and impregnated Phoebe. But ultimately he would be vanquished once again. His spawn however gained control over Phoebe in one final attempt to take over the underworld. The Seer used magic to transport the baby from Phoebe into herself so that she would be queen of the Underworld. However like Phoebe the baby had too much power and threatened to burn her out. But in an attempt to gain the respect of the council members by killing the Charmed Ones she tapped into the unborn child's power. As the Charmed Ones used their power of three spell the energy attack reflected back and destroy the Seer, the unborn child, and all of the Council members.
Final flesh appearance
In his final appearance on the show the original Source was resurrected with the power of Wyatt
. But was quickly sent back to oblivion through the vanquish of the demon that was his anchor to the physical realm.
The Source has once again appeared to destroy the Charmed Ones however this time he has appeared in the form as a Golem. Though he was quickly vanquished for what it seems like the final time with the "most powerful spell ever created." By the Charmed ones using the magic of every creature they've met as well as Excalibur.
As the leader of all evil the Source has multiple and powerful abilities. They include but are not limited to:
-Energy and Fire balls
-Teleporting (covers himself in flames)
-Resistant to upper level witches powers. (Presumably immune to lower level witches and other magical beings)
-Summoning (the ability to summon evil beings. Mostly used for to summon his minions.)