Shikigami (式神) are a kind of spirit summoned to serve a practitioner of onmyōdō, much like a western familiar. Shikigami cannot be seen by most people, but according to the Heian period onmyōji who were said to control them, shikigami often looked something like a child-sized oni demon. Although invisible, shikigami supposedly could, at the onmyōji's command, take a variety of human or animal forms, possess or bewitch people, and even cause bodily harm or death.
However, Seireitou's takes the form of eel-like spirits that when summoned, float and fly around him. He signed a contract with them while he trained with Rikudou Sennin in Sinsi, and trained to master their powers. While he summons a large amount of them, there are few specific ones that have bonded to him through-out his useage of them.