Seemingly, any demon in the underworld can become The Source, so long as they've proven themselves to the various underworld leaders and take part in a dark ceremony. The ceremony requires a dark priest and a spell read from the Grimoire, after which the demon will become "The Source of All Evil", leader of the underworld. In truth, however, the Source is in fact a demonic entity that possesses and merges with various hosts.
The First Source
Initially, the Source seemed responsible for all of the organized attacks against the Charmed Ones. He first appeared on screen in the Season Three finale "All Hell Breaks Loose", in which the Charmed Ones' magic was exposed on national television. He agreed to have his minion Tempus reset time in order to save magic from being exposed, but at a price; Phoebe Halliwell would be forced to stay behind in the underworld.
Knowing that Piper would be killed and Prue would soon follow, he planned to kill Phoebe and end the Charmed line. After the death of Prue, he went after the half-sister Paige Matthews, seeking to exploit an ancient law in the grand design that enabled a young witch to choose either good or evil. Going as far as to possess her boyfriend to lure Paige to the dark side, his plan was thwarted when Paige saw through his attempt and chose the side of good. However, he vowed still to break the Charmed Circle once again.
In this Source's final battle against the Charmed Ones, the Hollow was revealed - an ancient force that absorbs all magic. In 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 obtained Piper's powers, though later captured the abilities of Paige. In an attempt to summon their powers back, they inadvertently summoned The Source himself. A battle ensued and just as he was preparing to end them, Cole Turner appeared infused with the Hollow and absorbed one of the Source's powers. Using a spell calling forth the power of the Warren Line, the Charmed Ones destroyed this Source, and it would seem there struggle against him had concluded.
Cole Turner, the Second Source
It was, however, revealed that the Source did not truly die, but rather his previous host was destroyed. In a plan formed by The Seer using the Hollow, his essence was transported into the void left within Cole, whom was previously the half-demon Belthazor.
Initially, it seemed that Cole's love for Phoebe could hold the Source at bay. Overtime, however, the Source's essence began to take greater control, and he began to reorganize the underworld. His plan was to marry Phoebe in a dark wedding and produce an heir, a plan which succeeded. However, as suspicions over Cole grew among the Halliwell Household and the true nature of the Source within him was revealed, the Charmed Ones eventually vanquished Cole, thus destroying yet another host.
The spawn of the Source, however, attempted to take control over Phoebe in a final attempt to take control of the Underworld once again. The Seer used magic to transport the baby from Phoebe into herself so that she would be Queen of the Underworld, and eventually, the baby could be raised as its rightful ruler. However, the Seer experienced the overwhelming power the baby possessed, just as Phoebe had, and said power threatened to burn out of her. In an attempt to gain the respect of the demonic council by killing the Charmed Ones, whom are entrapped, the Seer mistakenly tapped into the Unborn Child's power. The Power of Three spell, however, made the Charmed Ones impervious to the child's power, and inevitably, the Unborn Baby's overwhelming might spiraled out of control and vanquished the Seer, the council members, and itself.
Final Live-Action Appearance
In his final appearance on the show, the original Source was resurrected with the power of Wyatt Halliwell, and anchored to the physical realm by a demon. However, he was quickly returned to oblivion when said demonic anchor was vanquished.
The Source once again returned in the comics following the conclusion of the show, this time in the form of a golem. Though formidable, he was vanquished for what appears to be the final time with a very powerful spell that used the magic of many creatures, as well as Excalibur, a potion, and a spell.
As the leader of the Underworld, the Source is perhaps one of the most powerful demons the Charmed Ones have ever faced. He is the bearer of many magical abilities and almost unparalleled influence throughout the underworld. His powers shown include (but are not limited to);
- Energy/Fire Balls
- Flame-Based Teleportation
- Mind Control
- Resistance to witch's powers