As noted in Dune, Rossak is the source of the Bene Gesserit's original poison drug, predecessor of the Water of Life.
The Legends of Dune series establishes it as the fifth planet of Alces Minor, and one of the stopping places for the Zensunni Wanderers. Its jungles are the source of many pharmaceutical riches; as the homeworld of the Sorceresses of Rossak, it is the origin of Bene Gesserit order itself and also the birthplace of Norma Cenva.
Read more about this topic: List Of Dune Planets