Karn was apparently born to Mannan and lived almost since the beginning of time. Along with his siblings, he served as a Rider of Mannan, doing the bidding of the Unseelie King. In the old days, he hunted giants and monsters with his siblings.
In September 2012, Karn and his siblings were sent after the Blackthorn family to retrieve the Black Volume of the Dead for the Unseelie King. Karn, Etarlam, and Eochaid went after Kit Herondale and the twins Ty and Livvy Blackthorn in London while the remaining Riders went to Cornwall for Julian Blackthorn.
The Hunter Gwyn ap Nudd and Shadowhunter Diana Wrayburn joined the ensuing fight with the children. The battle was interrupted when Eochaid, and later Etarlam, proclaimed that they had felt their brother Fal's death, with Karn only able to express his disbelief. Before leaving to rejoin their other siblings, he swore to make Gwyn pay for his treachery.
Karn and the remaining Riders attacked the London Institute shortly after. He was fearful of Emma Carstairs' sword Cortana, recognizing it as a blade forged by Wayland the Smith, but attacked Emma anyway and fought until Annabel Blackthorn appeared and threatened to reveal the Unseelie King's name, forcing them to retreat.
Like all the Riders of Mannan, Karn is arrogant and confident in his abilities. He has no problem killing and seems to enjoy carrying out his missions.
Karn, unlike his siblings, has short bronze hair in curls that make him resemble a Greek kouros. He is beautiful, with a cruel and lovely face covered in a bronze half mask, and wears the typical bronze cloak and armor etched with designs of waves and the sea. He also wears a longsword at his side.
- Longevity: As a Rider of Mannan, Karn is very old and has been alive for thousands of years.
- Enhanced strength and speed: Karn is able to move incredibly fast and deal heavy blows.
- Deceit: As a Rider of Mannan, Karn was wild magic and could lie despite being a faerie.
- Lord of Shadows (first appearance)