The War of Horrors
Symbionts are creations of the daelkyr, the lords of Xoriat. They are sentient organisms designed to bond with a host and function as a weapon or armor. The daelkyr armed themselves with these gruesome creations during the Daelkyr War, which ended thousands of years before the current era. Despite the daelkyr’s eventual defeat and expulsion from the surface of Eberron, scattered symbionts still exist in Khorvaire.
Although non-daelkyr can bond with symbionts, these items posses frightening, alien intellects which can drive a mortal mind to madness. The process is also permanent, as the symbiont becomes part its host’s anatomy.