Ending the eternal struggle between good and evil
Rennek was a formidably powerful Darklighter whom Leo Wyatt fought for decades for killing multiple magical practitioners. For vanquishing him, or so he thought, he was rewarded with the highly desirable job of becoming the Charmed Ones' Whitelighter.
Decades ago, Leo fought Rennek to kill him once and for all for murdering numerous witches and practitioners. It was mentioned by Leo that it took him decades to kill this darklighter, because he was too powerful. Leo eventually gathered enough strength to destroy him at the time. It is unknown if Rennek actually died when Leo "killed him" and he was brought back from the dead decades later, or if he was just assumed to be dead.
Sometime in 2008, Neena either resurrected or found Rennek. Rennek teamed up with Neena, whose true identity was still not known at that time. He helped Neena kill the Vampire Queen for making her upset. Unlike the other Demons in the Underworld, Rennek knows exactly who Neena is, referring to her by her many names (Isabel, Neena and Victoria).  Rennek acted as Neena's lieutenant when she and their demon army conquered The Heavens,  and captured his old enemy, Leo, in revenge. However, Leo got the upper hand on Rennek and escaped. After Neena was defeated, Rennek fled without a trace . Rennek then began working on a plan for the Charmed Ones to destroy their family themselves.
Plotting against The Charmed Ones
He acquired the Jewel of Orthon from Alexi, a Seer. With it, he was able to take control of Montgomery Knox, the headmaster of Knox Academy, a neutral alternative to magic school. To suit his means, whatever they might be, Rennek had Knox attempt to recruit Tyler Michaels, a firestarter and friend to the Charmed Ones. Additionally, he has also taken control of Bailey, a young Magic School faculty member and had her convince Paige to recruit a neophyte witch named Sarah, who is currently being mentored by an older witch named Patience.  Patience, is actually Prue, the deceased oldest Charmed One. Rennek apparently knows that if Prue reunites with her three sisters, the Power of Three will be broken, and the union would cause other cataclysmic results.
After the four sisters were united and the Power of Three went out of control, Rennek took this opportunity to capture Leo. He revealed that his ultimate plan was to distract the sisters just enough so that he could get to his old foe. As a result of being a Whitelighter, an Elder, and an Avatar, Leo has subconsciously acquired massive amounts of prominent information on the magical community over the years and Rennek uses an ominous looking ally to get it and flees before the Charmed Ones arrive. He is later seen outside of Halliwell Manor, revealing that it wasn't over. 
Taking over The All
Rennek exploited recent events, such as Prue's return and Neena sharing of the Source's power, to exploit the magical unbalance. Impaling the Empyreal Sword in the Grimoire, Rennek was capable of taking over The Nexus of the All, the basis of all magic. With the All under his control, Rennek cut the Earth off from both the Heavens and the Underworld, putting an end to the Good vs. Evil fight. 
However, a side effect of this action was that magical beings were stripped of their powers and mortals were given access to magic. While the magical community was left to deal with the consequences, Rennek enjoyed luxury within the Nexus of the All. 
However, the Power of Three was not destroyed, but instead went to the only available candidate, Prue Halliwell. With the powers of all four Charmed Ones at her disposal, Prue sought to discover the source of the magical crisis, eventually finding out about Rennek's actions. Realizing the Halliwell sisters were on his tail, Rennek used the new magic to launch a massive attack on the Halliwell Manor. Prue used all her new powers to send the army away at the cost of her own life. Fortunately, using all her new magic caused the Power of Three - and the Warren Powers - to return to their rightful owners and Paige was able to heal her just in time. 
Powers and Abilities
As a Darklighter, Rennek is one of the most powerful magical beings shown so far, not only possessing the typical darklighter powers but also being in possession of various other supernatural abilities, bestowed by him when the first witch Neena used a spell to grant the newly vanquished Source's powers to all evil beings.
- Darklighter Powers
- Black Orbing: The power of teleport through the use of black orbs.
- Apportation: The power to transport an object or person through planes from place to place. Rennek use this power to materialize a darklighter crossbow, whose poison is lethal to all whitelighters and Elders.
- Sensing: The power to sense the locations of beings and people within a plane of existence.
- Voice Manipulation: The ability to alter one's voice to sound like another or add an echo to one's voice when angered.
- Touch of Death: The power to automatically kill a being upon contact. Rennek used this power to kill one of his "magical assistants" due to his constant speaking of duties.
- Other Powers
- Immortality: The power to live forever with immunity to diseases and with arrested aging.
- High Resistance: The power to be resistant to other beings' powers and magic.
- Energy Beam: The power to shoot deadly beams of energy, usually through the hands, at an intended target.
- Energy Magic: The power to manipulate and project energy into a desired form. Rennek used this power to create a prison bars, a sword and metallic chains out of purple, shadow-like energy.
- Reality Warping: The ability to change reality and the world to various degrees. Rennek obtained this power by taking over the Nexus of The All, the source of the world's magic, through the power of the Grimoire and the Empyreal Sword. With this he was capable of creating a paradise island for himself as well a platform with magical hologram from which he could spy on the Charmed Ones.
- Telekinesis: The ability to move objects and beings with the mind. Rennek used this power to push Leo back.
- Shielding: Rennek used this power to shield the Sword and the Book.
Notes and Trivia
- Rennek killed the first charge Leo ever lost, Nancy.
- He and Leo fought for five decades, from the 50s to the 90s.
- Despite being evil, Rennek seemed to not share the typical atittude of dominion like past evil forces the Charmed Ones faced. Instead he seemed bored of the eternal struggle between Good and Evil and worked to end it.
Rennek has appeared in a total of 16 comics throughout the course of the series.
|Charmed, Season 9|
|#01||"Charmed Lives"||#09||"The All or Nothing"||#17||"Family Shatters"|
|#02||"No Rest for the Wicca"||#10||"Three Little Wiccans"||#18||"Four's Company"|
|#03||"Innocents Lost"||#11||"Last Witch Effort"||#19||"Crossed, Triple-Crossed"|
|#04||"Mortal Enemies"||#12||"The Charmed Offensive"||#20||"The Old Witcheroo|
|#05||"Unnatural Resources"||#13||"Piper's Place"||#21||"Reversal of Misfortune"|
|#06||"Morality Bites Back"||#14||"Cupid's Harrow"||#22||"Prue Ya Gonna Call?"|
|#07||"The Heir Up There"||#15||"Where There's Smoke There's a Firestarter"||#23||"The Darklight Zone"|
|#08||"Oh, Henry"||#16||"The Heavens Can Wait"||#24||"The Power of 300"|
|#00||"To The Warren Born"|