He is another one of the Fiend's offspring, an abhuman. He considered Lord Barrule his master and controlled the buggane for a part.
His first and only appearance is in Rise of the Huntress when Alice and Tom go into a tavern to find a ship that will take them to Ireland. Commander Stanton enters the tavern with several yeomen as well as Horn chained up in between. Horn recognizes Alice to have darkness inside her and Tom to have a sliver of darkness inside him.
He continues to make small appearances throughout the book but his notable ones include helping Tom and his company escape the tunnels, his reasons being wanting revenge for his master's death. He was later chained up by Lizzie in a cell along with Stanton waiting to have his energy sucked out and be killed. Later Tom gets put in the same cell. Horn then according to Tom's plan, breaks open his own shackles and those of Tom's with his immense strength. They escape the dungeons and go up the stairs to confront Bony Lizzie. Lizzie angrily comments that Horn has crossed paths with her one too many times and then causes Horn's head to implode using her magic.