Because he was the twin brother of Melaka, Harth inherited the Slayer's natural precognitive abilities, thanks to which he experienced dreams about the lives of previous Slayers, and also gave him the instinctive knowledge of how to become a vampire when he was attacked by Icarus. Thanks to his abilities, he became known as The One Who Will Lead, the undisputed vampire master of Haddyn.
For four years, Harth gathered a number of talismans through Gunther. These talismans would allow him to perform a ritual to raise the Old OneNeauth, whose womb would serve as a gateway between Earth and a demon dimension. This action did not sit well with two of the Old Ones: Boluz and Vrill, who hoped to unleash apocalypse but on their own terms and schedules. They sent the demon Urkonn to train Harth's sister in the ways of a Slayer, and she subseqeuently defeated Harth's attempt at ending the world.
Later, Harth came to ally himself with the mysterious "Black Hope": a centuries-old Willow Rosenberg who had once again gone "dark." Driven insane, and not in access to the totality of her power after the end of magic, together they conspire to bring 21st century Slayer Buffy Summers to the future; Willow tells Harth that doing so will end their reality, but she tells Melaka that Buffy's return to her own time will end their reality. In the end, he attempts to rebel against Willow but she ensured that Gunther would seek vengeance on Harth after his failed assassination attempt. With Harth only playing as a minor pawn, Dark Willow is killed by Buffy who returns to her own time, and Haddyn appears to continue existing.