A Dark King is the descriptive title by which Bo's Dark Fae biological father was considered to be for four seasons. He has also been referred to as the "Lord of Darkness" and the "King of the Underworld". In It's Your Lucky Fae, Bo specifically identified her father by the name Hades.
In Season 1, it was established that Bo's Light Fae birth mother, Aife, was sentenced to death and handed over to the Dark Fae for execution after rebelling against the truce that ended The Great War between the Light Fae and Dark Fae clans, and killing a leader of the Dark. However, instead of capital punishment she was imprisoned for several centuries by an unidentified Dark King and subjected to torture. During her imprisonment she was raped by him and became pregnant with Bo. Aife managed to escape from her cell after giving birth to Bo and thereafter Lou Ann, her midwife and a Dark Fae rebel, fled from his kingdom with Bo in her arms, leaving her with a human family to be raised in secret.
Since learning that her father was a cruel and monstrous Dark King, Bo has feared that she, too, could be a monster just like him.
While inside The Temple, Bo saw a vision of her father sitting in a chair and holding her in his arms while singing to her. Bo only saw him from his back. He then proceeded to leave the room and left Bo in the care of a nurse maid, who was then killed by Aife and the infant Bo snatched away. This vision of a loving, tender father changed some of the perception Bo had about him.
When Bo found herself in the middle of a coven of human witches a powerful entity intervened, possessed one of them, spoke to Bo through her, and told her that her power was beyond her own comprehension, and that soon the two of them would rule together. (The assumption made was that it was Bo’s father.) (Adventures in Fae-bysitting)
While Aife was in her cell she said in a crazed state that if Bo's father knew about Dr. Isaac Taft and his laboratory he would kill and resurrect the humans over and over again as retribution for capturing, torturing, and experimenting on Fae (Those Who Wander).
In the finale episode of Season 3, Tamsin and Dyson are en route to The Dal and a figure appeared in the middle of the road with his back turned towards them. "The Wanderer" song then began to play on the car radio. The allegation that he was Bo’s father was made by Tamsin at that moment, but it was not confirmed.
In Destiny’s Child, we find out that Rainer is not The Wanderer and, therefore, not The Dark King and Bo's father. Additionally, in Waves, Rainer tells Bo that he was not the one who had captured her in black smoke and transported her to the Death Train. In End Of A Line, when Rainer's face materializes on a blank page of a book, Tamsin doesn't recognize it and Bo tells her that it's the face of her "boss"; to which Tamsin responds that the face in the book is not him (removing all possibility that Rainer is The Wanderer that hired Tamsin to find Bo).
When the Cinvat (portal) to Hel was opened after the handfasting ceremony between Bo and Rainer in Origin, the Lord of Darkness was released from his prison. As Bo was feeling his attempt to cross the bridge and surface, she asked Trick to tell her all he knew about her blood and Trick swore that the true identity of her father remained shrouded from him. (Dark Horse)
The Lord of Darkness — the Pyrippus — is stopped from coming through the gateway by Kenzi, who sacrificed her life to save Bo from death by walking through the Cinvat. Kenzi was Bo's "heart" and her death fulfilled the prophecy.
In Like Hell Pt.2, he was revealed to be Hades and ruler of Tartarus. In It's Your Lucky Fae, Bo revealed that he was her father; followed by his showing himself to her for the first time in End of Faes.
His species and range of abilities have not been established.
From what Aife said about him while she was a prisoner of Dr. Isaac Taft, he is vengeful and punishing.
He has the power of resurrection (mentioned by Aife in Those Who Wander).
He seemed able to possess Bo whenever she was in great peril, which indicates he could be telepathic.
- "My father was most certainly dark, but what if he was a monster? What does that make me?" – Bo to Trick (Caged Fae)
- "I will reign as he did, for I am his daughter. Together, we will bridle the masses and ride into victory. Even death will fear us. Only I will choose who lives." – Dark Bo (The Ceremony)
- "You know not your true strength my child but soon you will and the world will bow down before us." – (Voice) (Adventures in Fae-bysitting)
- "If your father was here, he would kill them all! And then resurrect them and then kill them again!" – Aife to Bo (Those Who Wander)
- "I am your Queen, whether you swear it or not, fool, and my true army cometh. I was bound by blood. I will bathe in it. Humans, Fae...all will bow before me. All will break beneath the power of the Pyrippus." – Dark Bo (Dark Horse)
- "I was born in Hel. I found out when I was in Tartarus. My dad is Hades." – Bo to Tamsin (It's Your Lucky Fae)
- The term Dark King was used by Aife in Blood Lines to describe her captor, tormentor, and abuser — and by implication, Bo's father. After this episode, it was not used again in the series.
- Some fans of Lost Girl believed that if The Wanderer was Bo's father he may be the Norse god Odin, because he is also known as "The Wanderer". This suspicion was supported by the use of Tamsin, a Valkyrie, as the mercenary sent to ensnare Bo. (In Season 4, the identity of The Wanderer was confused with that of Rainer because the latter was also referred to as a "wanderer" for being cursed to ride the Death Train infinitely.)
- In The Ceremony, the painting Trick unrolls shows the figure of a giant horse with bat wings hovering over a group of people and bearing down on them with what appear to be flames, (or perhaps drawing energy from all of them as Dark Bo has done). It is an illustration of the Pyrippus. Although the name itself does not appear in Greek or Roman mythology, "Pyrippus" might refer to one of the four horses that pulled Hades' chariot in the underworld. It may be an alteration of Pegasus, the flying horse in Greek mythology; or a variation of the Gryphon.
- The term "Lord of Darkness" was used for the first time in Origin, the twelfth episode of Season 4. "King of the Underworld" was used for the first time in Like Hell Pt.2, the second episode of Season 5.
- In Origin, the image of the Pyrippus represented the Lord of Darkness. In Dark Horse, the Lord of Darkness was established to be Bo's father. In Like Hell Pt.2, his name was revealed to be Hades.