Harmonthep was a swampy world with a nitrogen-oxygen atmosphere. It was also rich in biological life. Consequently, it was also capable of accommodating human life, and for a while was the home of a population of Zensunni. Inglsey noted Harmonthep as the sixth stop in the Wanderers' migration.
At some point prior to the Butlerian Jihad the planet was one of the Unallied Planets, housing Zensunni tribes; at one time, such a tribe was attacked by Tlulaxa slavers. Little Ishmael was taken by the slaver Tuk Keedair and sold at Poritrin.
After several years, Ishmael and other slaves stole the prototype spacefolder vessel from Poritrin. The ship crashlanded on Arrakis and Ishamel and his people joined the Wormriders of Selim. Ishmael married Selim's widow and was the first to call these people the Free Men of Dune.
Harmonthep was remembered as one of the origins of the Fremen. It was eventually totally destroyed.