Desians is a group hailing from the GameCube RPG, Tales of Symphonia. Desians refers to a group of evil oppressors in the land of Sylvarant. They are technologically advanced, operating from their Human Ranches. Any prisoners they take end up in the human ranch as their slave workers.
Desians tend to use up most of the worlds Mana creating their technology. They are said to be wiped out by the Goddess Martel when the The Chosen of Mana regenerates the world by completing her pilgrimage.
Desians are actually a tool used by the church of Martel, or more specifically, Cruxis. The church and the Desians are one and the same. Whenever The Chosen of Mana completes his or her mission, the Desians and their evil acts of oppression are moved to the other world, Tethe'alla. Their purpose is to impose despair on the world so that they are not able to use up as much Mana.