Julian McMahon appeared on the U.S. TV series "Charmed" as the Assistant District Attorney, whose alter ego was which powerful demon?

Within the 1998 - 2006 American TV series "Charmed", Assistant District Attorney, Cole Turner is played by Julian McMahon. He is a half-human and half-demon by blood and was better known to the demonic world for over a hundred years as the legendary demonic assassin Belthazor. Cole is dispatched by the "Source of all Evil" to kill the Charmed Ones (three powerful good witches). Cole however falls in love with the youngest sister Phoebe (Alyssa Milano). He will renounce evil and become a close ally of the good witches (sisters). Though Cole eventually rids himself of his demonic nature and marries Phoebe in the fourth season, he never gains the trust of all of the sisters.

In season 4 of the show, Cole returns to being evil. He unwillingly becomes the new Source of All Evil. As The Source, he is vanquished by The Charmed Ones. He comes back from death and attempts to win back Phoebe's love. Driven insane, he will once again in season five be killed by the sisters. He later returns in a guest appearance in the season seven episode: "The Seven Year Witch". In this episode, it is revealed that his spirit is trapped between two completely different realms. Cole claims that he is now in limbo to atone for his sins.

Cole presented a dualism in the show. He had a mind with consciousness and self-awareness. It was distinguished from his brain, which was the physical seat of his intelligence and power. He realized that he had to be a man and not a demon to find love.