Shikigami or Shikijin were fierce gods employed by the Onmyoji (Master of Yin yang) to judge the evil doings and well doings of the populace, and they were also called Shiki no Kami, and in some literature, Shiki or Shiki Kijin.
Shiki' means 'to use' and to employ.
Kijin (fierce god)
It refers to malignant gods and specters.
It is a Reikon (Departed soul.)
In Buddhist terminology, it is a supernatural existence that causes mysterious phenomena.
Onmyodo (yin-yang philosophy)
Onmyodo is a combination of Shinto (Ancient Shinto) and the Inyo Gogyo Shiso (Yin-Yang Wu Hsing Ideal) of Taoism and the philosophy of mysticism, and strains of this can be found in the present day Shrine Shinto theology and ceremony.
When the Kannushi (high priest) and Miko (high priestess) summon the god and allow themselves to be possessed by the god through Kagura (Ancient Shinto music and dancing) and Kito (prayers,) they invoke the divine spirit of Nigimitama (spirits of peace), but since Shikigami is said to have been a fierce god, it is believed that Onmyoji employed not only the divine spirit of Nigimitama but also the divine spirit of Aramitama (violent soul,) that is a 'violent god' and 'the gods of lower status who morphed into specters and monsters.'
Observing the good and evil within people, one sees the movement of people's hearts (souls) between Nigimitama and Aramitama, which is why Shikigami who possessed the two different spirits were employed.
Izanagi-ryu, a form of Onmyodo that is handed down in Kochi Prefecture, Shikoku, is called 'Shikiouji.'
In most literature and tales about, or involving an Onmyoji, the Shikigami that the Onmyoji employed usually appears in the form of a piece of washi (Japanese paper) that morphs into a bird or beast according to the will of Onmyoji, but the "Fudo-riyaku-engi" describes the Shikigami as an anthropomorphic vessel (tool) shaped as a bird or cow, much like the specters and demons throughout the Muromachi period to the Edo period and it depicts the Shikigami as a violent god.
Ushi no Koku Mairi
There is an occult art called Ushi no koku mairi that has continued from the Heian period, which one would drive a nail into a sacred tree, become a demon and invoke a curse on a hated enemy. By driving a nail into a sacred tree at Ushi no koku (midnight from 1 am to 3 am,) it was believed that the barrier between worlds would be broken and the evil-wreaking gods (devils and specters) would be summoned from the Tokoyo (eternal night kingdom of God), possessing the summoner and cursing whoever they hated. Ushi no koku mairi and Onmyodo were much the same in that they would summon specter and have them place a curse.