Cole Turner is a demonic villain from the TV series, Charmed.
He was born in California on January 19th, 1885 to a human male and an upper-level demon. His father found out what she was and tried to take him from his mother, but she killed him and raised him to hate his human side. He then took on the name Belthazor and became one of the most dreaded and most powerful demons in history. Later on, The Source chose him to be their link to the outside world and had his mother put him through law school which the end result was him becoming a lawyer. The Source also made him apart of an elite demonic society called the Brotherhood of the Thorn by 1901. After that he built up a reputation for himself to be a stone cold assassin, mainly killing anyone with an athame.
The Triad's Assassin
A demonic group known as The Triad were getting desperate in their war against The Charmed Ones. So they hired Cole to destroy them once and for all. They chosed him because he was half human and could get close to them. He set himself as San Franscisco's District Attorney. He first met the Charmed ones on a case which they were a witness to. During this time, Phoebe Halliwell, the youngest of the Charmed Ones fell for him. He decided to use this to his advantage. He manipuated them through her, however the elder sister, Pruedence Halliwell, did not trust him. Though later on, he fell in love with her and found his humanity. He could not kill them and revealed his true nature to Phoebe and the Halliwells. . After he refused, The Triad was going to kill him, but he killed them instead.
On the run & Becoming human
He was now a rogue demon. He aided the Charmed Ones in vanquishing demons while he was on the run from the forces of The Source. He proved to be a valuable ally to The Charmed Ones. He helped to clear Prue's name after she was set up for a murder she did not commit which lead to not only Prue to be free, but it also helped the marriage between the middle sister, Piper Halliwell and Leo Wyatt to go forward.
Sadly, he could not prevent the death of Prue, a death which broke the Charmed Ones. He gave Phoebe moral support. Meanwhile, they discover a forth sister or rather half-sister Paige Matthews. Cole helped Phoebe, Leo, and Piper to protect her. The Charmed Ones were reformed and the sisters killed the demon who killed Prue. After a few more battles, they were finally able to strip his powers with a potion, turning him human in the process. Though his transition from demon to human proved a bit of a challenge for him, but he overcame it. Sometime later he proposed to Phoebe and she accepted. However later on, The Seer, a demon who works with The Source betrayed her master when he tried to unleash a mystical equivalent of a doomsday device known as The Hollow as the final solution against The Charmed Ones. With The Seer's help, Cole assisted the Charmed Ones once again to defeat The Source, once and for all. However, there was a price, he becomes a demon once again, not just any demon, The new Source.
Being The Source & Death
Cole would confront The Seer after, however she told him to embrace his destiny. He tried to fight it, but ultimately he could not and became evil once more. He slowly started to gather forces. But as far as the Charmed Ones were concerned, he was still mortal. But Paige was somehow able to see right through him. Though she could not put her finger on it, something about Cole did not feel right. Never the less after some manipulation from The Seer, he married Phoebe in an unholy wedding. Because of this act, The Seer's plans started to come to fruition, Cole and Phoebe would later on conceive an evil child.
After a series of events, Piper joined Paige in suspecting Cole of possibly being a demon again. Shortly after, Phoebe discovered she was pregnant and after a battle with an evil wizard who tried to take Cole's powers for himself, Cole revealed himself to the sisters after Phoebe(who was herself turning evil as the result of her pregnancy and some manipulation from The Seer) helped him. The Charmed Ones were once again broken and Cole and Phoebe became The Source and Queen of the Underworld.
Cole and The Seer were giving Phoebe this evil tonic to make her stay evil. However, she still desired to help her sisters. But while helping her sisters, she let one of the demons lived. This created a dilemma for Cole. This could be shown as a sign that they are working against The Underworld from the inside. Phoebe would also find out the tonic was making her evil. However, after some convincing from Cole, she drank the tonic. But then after he said he had to kill the innocent she saved which prompt Phoebe to induce vomiting to get rid of the tonic. Shortly after to avenge the innocent, Piper, Paige, and Leo confronted Cole. Both sides tried to convince Phoebe to help them. With reluctance, she helped her sisters kill Cole.
Resurrection, Suicidal, Stalking Phoebe, Final Death, & Afterlife
After his death he tried to contact Phoebe. He got Phoebe to come down to the demonic wasteland and tried to get her to resurrect her with evil magic. She refused and left Cole, leaving him hopeless. he discovered he could take the powers of other demons. He returned to the land of the living and saved Phoebe from an evil witch hunter. He swore that he was not going to give up on her. To Phoebe's chagrin, he stalks her, he makes her life a living hell. He tries to do good, but it did not impress Phoebe enough. She wanted him out of her life.
After a series of failures to win back Phoebe, he tries to kill himself. He tries various methods, even once he even tried to kill Paige's father to prompt the Charmed Ones to kill him. Nothing worked. However, he unintentionally succeeded when he became an Avatar and changed certain events in history(one of which was The Source killing Paige rather than her becoming a Charmed One) so that he can be with Phoebe. However Phoebe did not love him in this reality either. Also Paige after an accidental orbing was in this new reality. She reunited the Charmed Ones and Leo and together, they vanquished Cole, for in this reality, he was not invincible.
In this final death, he wound up in purgatory. A void-like plane between life and death. He still had some influence in the demonic underworld. He convinced an honorable demon, Drake, to become human for a year but he would still have his demonic powers. Though after that year's end he would die. With his help and a planned demon attack on Piper, he helped Phoebe to not give up on love. He succeeded.
In the end, Cole moved on and accepted his fate, but is rest assured that his one true love, Phoebe would not share his fate.