Horphis is a visitor in Great Kourend's Arceuus House Library. He seeks dark manuscripts within the library, and will ask the player to retrieve them once they have at least 20% favour with the Arceuus House. He will give players a book of arcane knowledge for each manuscript turned in, in addition to 5% favour.
Training Magic by collecting Dark manuscripts and trading them for books of arcane knowledge could yield around 35,000 Magic experience per hour if their locations stay the same during that time.
Horphis is the solution to the medium anagram clue: HIS PHOR. The answer to his challenge scroll is 1.