The Desians are a group of half-elves in Tales of Symphonia, and serve as the primary enemies of the protagonists for much of the game. Viewing all other races as inferior, they set up large human ranches where they cultivate humans to turn into Exspheres. Each ranch is headed by one of the Five Grand Cardinals.
The Desians are a group of half-elves that carry out Cruxis' will in the declining world. On the outside, they go around breaking havoc on the humans, often taken them as prisoners. While these humans believe the Desians are evil, Cruxis is seen as a holy organization for the religion Church of Martel, and nobody knows that they are in fact the same.
However, as the story goes on, it is revealed that their real purpose is to enhance the Chosen's Cruxis Crystal by deliberately putting him or her in harm's way, as Cruxis Crystals and Exspheres only grow when the host is under stress or in danger. However, they are told not to kill the Chosen, because the Chosen is the primary factor for Cruxis’ goal.
The ultimate goal of the Desians is to begin an era of half-elf superiority called the Age of Half-Elves, led by Cruxis and its leader, Yggdrasill. It is unknown if the common Desians know that they are lead by the holy organization, but it is confirmed that the Grand Cardinals know their leader is Yggdrasill, and Pronyma, Forcystus and Rodyle have confirmed their knowledge of the organization. Magnius also mentions Yggdrasill as he dies.