Symbionts are the creations of the daelkyr, the lords of the plane of Xoriat. They are sentient beings that are designed to attach themselves to hosts, and act as weapons and armor for their hosts. Since the end of the Daelkyr War thousands of years ago, symbionts still remain scattered across Eberron.

Eberron Pathfinder rules page.

A symbiont can exist on its own for a length of time; however, it will seek a host as soon as one becomes available. When a symbiont bonds with a host, the symbiont immediately tries to take possession of the host, enforcing its will onto the hosts’. Only those with strong wills can overcome the mental invasion of the symbiont. Once the symbiont is subdued the first time, the symbiont will usually act cowed, subjecting itself to the users wishes. However, the symbiont may try to take over its hosts mind again if the host is especially weakened, or if the host tries to do something not in the best interest of the symbiont, like remove it or attack another symbiont. When the symbiont gains dominance, the symbiont is in full control of the host’s actions, though it usually becomes apparent that the host is no longer in control of his or her body. Even in a cowed state, the symbiont may subtly influence its host, fanning the fires of rage or increasing a host’s hunger for its own nourishment. Some hosts can be driven mad by the constant whispering of a symbiont in its mind.