The Source of All Evil is an evil essence and the ruler of the Underworld. Millennia ago, the Source was one of the ancient and powerful beings that ruled the world. When his kind was cast into purgatory, the Source left his physical form and possessed a human, thus becoming the first demon known as the Source of All Evil.
The Source has had many incarnations, as each time his physical form was destroyed, the essence would possess a new host and assume its personality. This process is unknown to most demons, as they believe the Source is merely a title held by the current leader of the Underworld.
When Adam sought to open the gateway to purgatory for the rest of his kind to return, he kidnapped Cole Turner, as he required his blood as a former host of the Source. Adam then revealed the truth about the Source's origins. The Charmed Ones came to is rescue and used his blood to open the portal to send Adam back. As he was too powerful, Cole sacrificed himself and they were both sent to purgatory.
When the Adam crisis was over, a former host of the Source was able to claw her way out of purgatory as reclaimed control of the Underworld. Freed from the essence, she claimed the title Mother of All Evil.