Harum was the dominant personality in the council of personalities developed by Leto Atreides II to manage the tumult of his inner lives. It is unclear whether by accepting the presence of Harum in his consciousness above the presence of his other ancestors, Leto became abomination. He comments that by the judgement of the sisterhood, he may have done.
Harum is said to have been a king in ancient Earth, one who established a dynasty lasting some three thousand years. He was presumably an Egyptian since Leto's style of ruling is referred as the pharaohs' model. Alternatively, he may have been an ancestor of Hammurabi, whose line established the First Babylonian Dynasty of Ancient Mesopotamia. Leto explains the history of Harum to his Royal Scribe, Farad'n of House Corrino, in the final chapter of "Children of Dune":
"I'm a community dominated by one who was ancient and surpassingly powerful. He fathered a dynasty which endured for three thousand of our years. His name was Harum and, until his line trailed out in the congenital weaknesses and superstitions of a descendant, his subjects lived in a rhythmic sublimity. They moved unconsciously with the changes of the seasons. They bred individuals who tended to be short-lived, superstitious, and easily led by a god-king. Taken as a whole, they were a powerful people. Their survival as a species became habit."