This will be very brief as there is far too much to go into. The practice has a contentious history in Japan and was outlawed for some time along with its close cousin, Shugendō (修験道).
This is nowhere near an exhaustive study on the subject, I only hope to encourage your curiosity and spur you to study more. There is simply too much to cover and if I missed something you really hoped to read about in this post, don’t hesitate to comment or reach out to me about it.
In the 5th/6th centuries, the principles of Yin-Yang and the use of five elements were transmitted to Japan from China along with Buddhism and Confucianism. These esoteric practices combined with local religions to create what came to be called Onmyōdō. By the 10th century its practice had been well established. But by the 19th century it had been made illegal to practice.
Because it was later outlawed (1800s) many of its practitioners went into hiding and other sects were completely obliterated. As it was mainly an oral tradition what has been handed down to us is broken and many of the surviving rituals and records have been recreated based on the few written accounts found in texts left behind by the Heian period practitioners. The Heian period was the heyday for Onmyōdō, it was extremely popular among nobles and the court people. A good Onmyōji could make a wealthy living there. A practitioner of Onmyōdō is called an Onmyōji (陰陽師). And the most famous of these is the Heian magician, Abe no Seimei (安倍 晴明).
Abe no Seimei (安倍 晴明)
There are various legends surrounding his birth but the most famous of them is that his mother was said to have been a Kitsune named Kuzu No Ha(葛の葉) who famously fell in love with her human husband after he had saved her from being captured. While his father was a priest who was fluent in the arts of Taoism and Buddhist esotericism (Vajrayana/Tantric Buddhism).
The Tsuchimikado (土御門) family, descended from Abe, continued to be famous diviner’s working high ranking positions within the Imperial courts of Japan well past Abe No Seimeis’ death at 85yrs of age in 1005.
Abe was famous for being able to accurately predict the sex of children before they were born. Many Heian noble-women would come to him in the hopes that he would give them news that a male heir was due to be born.
His fame in the court grew and the court ladies often had fun by hiding objects and watching Abe find where they had been hidden via his skills on divination. He would sit in meditation and focus his energy on finding the hidden or missing object. And he was so accurate that he was eventually appointed by the court to help investigate strange occurrences and disappearances.
He often read the stars to determine the events of battles. And seemed to have a knack in geomancy (including positioning of the home such as in Feng-Shui) and for being able to locate where to tap the ground for digging wells or placing barriers.
He was in charge of creating wards, most well known in the form of paper Ofuda that are blessed by Priests and meant to keep demons at bay. Many types of Ofuda exist and they can be used for a variety of blessings.
Abe also made famous the use of the Seimei Kikyō (晴明紋) a mystical symbol of the five-pointed star referred to in the West as a pentagram. It was developed from the Tao practice of drawing a Wu Xing (五行) chart for divination via natural elements and Abe is credited with its creation as a magickal element in ritual.
The development of the pentacle/pentagram originated in the use of the Wu Xing chart of elements. It was popularized by Western and European travelers who were attracted to the magical Eastern practices and their unusual, but effective, methods. The variation of elements used in Eastern vs. Western magic are of little consequence as they were based on the availability and use of those elements at the time.
The Onmyōdō practices we are most familiar with today were established by Yakou Tsuchimikado (土御門 夜光) who, in the face of growing numbers of fake Onmyōji, decided to put a system in place to prevent this. Yakou Tsuchimikado created the Imperial Onmyōdō system that would allow the court to regulate and train professional Onmyōji. However, this also eventually led to the downfall of its practice when the Shinbutsu-Buri Law of 1868 (centuries later) demanded a separation of Buddhism and Shinto. By this point the practice had fully integrated aspects of Buddhism, Shinto, Taoism, and Confucianism. The Emperor frowned deeply on the strange practices and superstitions found working within Onmyōdō and other esoteric branches of Japanese religions and sought to abolish them, believing them to be archaic fear mongers.
After this, Imperial Onmyōdō quickly fell apart and the Tsuchimikado family was even disowned by the court and stripped of their rank and wealth. Despite this, elements of Onmyōdō continue to be found mixed in with modern Shinto and within folk practice such as many of the rituals found in Shugendō. The enigma of the practice continues to be a popular subject within media such as film, manga, anime, and games.
At its core, Onmyōdō is the art of divination. Onmyōji were adept at reading the stars and deciphering cosmological omens and movements. Many of the Ofuda talismans you see include star charts (dots connected with straight lines) on them or constellations. These are meant to invoke the power of the Kami or energy associated with that series of stars.
Onmyōji claimed to be able to speak with spirits of the human dead as well as Yokai. As experts in divination and spirit talk they were relied on by the Court to read cosmological movements to determine when ceremonies should take place, what days were lucky or unlucky, as well as predicting births/deaths, warding against evil spirits, communing with spirits/deities, and even helping with decisions of military movements and the placement of buildings.
However, their most famous activity was that of the exorcist.
An Onmyōji was capable of expelling demons from a human host as well as being adept mediums in their own right. Most exorcist would use a young woman to act as the host of a Yokai or Yurei (ghost) so that the entity might speak so that all could hear. The Onmyōji would ask a series of questions to determine the identity of the offending spirit so as to determine how best to expel it.
In one famous example, a young lady was thought to be possessed by an evil Kitsune spirit. During the questioning she became somewhat restless and anxious, climbing the roof of her home and barking so that the Onmyōji was forced to go after her. Using specially made Ofuda the Onmyōji expelled the Kitsune while the girl was being rescued from the roof. When the Ofuda was placed on her forehead, she immediately vomited black bile and regained her senses, the evil spirit now exorcised.
Many years were spent learning to use the protective spells and hand Mudra necessary to protect against possession. Their primary practice in this was the performance of Kuji-in/Kuji-kiri (use of magickal Mudra).
The Kuji-in/Kuji-kiri (九字切り) are a type of hand Mudra (often depicted in anime) that are meant to empower your chakra energies and provide enhanced protective auras. In magickal battle, they are used to strengthen the wielders offensive and defensive capability both physically and spiritually.
In Japanese, the nine syllables are:
Rin (臨) – The strength of mind and body
Kyō (兵) – Direction of energy
Tō (闘) – Harmony with the universe
Sha (者) – Healing yourself or others
Kai (皆)- Premonition of danger
Jin (陣) – Reading the thoughts of others
Retsu (列) – Mastery of Time and Space
Zai (在) – Control over the elements of Nature
Zen (前) – Lighting and speed
After these cuts are made the syllable Kō (行) is often intoned loudly but is not required. They can be used individually for specific focus of energies but when all 9 cuts are used together, they create a powerful grounding of the self and all its parts. It was the ultimate protective spell which came in handy as part of an Onmyōjis’ repertoire was the controlling and commanding of spirits to do their will. These Mudra were often combined with the creation and use of Ofuda specifically written with spells on them for protection and exorcism.
These were not the same as the Ofuda that would have been kept on a home Kamidana (altar).
Taizen Fukan No Sai (泰山府君祭)
A powerful spell said to protect from death, grant longevity, and even raise the dead. Though it was Abe No Seimei himself that created the spell, it was Yakou Tsuchimikado who made the ritual infamous. He was said to have used the spell to resurrect himself and extend his life.
This spell is unique in that it does not follow a specific formula. Instead, the Onmyōji will sit before his altar and intone the names of all the major Jigoku Kings (Hell Kings) – Lord Taizan and Lord Enma along with others: inviting them to dinner essentially. The Onmyōji would present a scroll with his offering outlined as well as his expectations in return. Often times these offerings involved human lives. This practice was outlawed and, naturally, is still outlawed to this day. This is the very same ritual used by the father of Hyakkimaru in the anime Dororo in which the various body parts of Hyakkimaru have been offered up to each of the great demon lords in return for power.
Shikigami are spirits who are summoned or created using complicated formulas and spells. Usually an Onmyōji creates a Shikigami as a type of servant, similar to homunculi. But a more accurate description would be that of a witch familiar. The Shikigami works to protect its master and often performs basic chores such as sending spirit messages, scaring away potential intruders, and acting as an additional source of energy for spells and rituals.
Generally, a Shikigami is either an enslaved spirit or a perhaps a thought-form. Of course, having an enslaved spirit is highly frowned upon unless the Onmyōji and the spirit have entered a contractual agreement – that was not always the case and there are many stories of young/inexperienced Onmyōji who were torn apart by their Shikigami servants after losing control of them.
Abe no Seimei is often depicted with one or more Shikigami, he was once recorded as saying that he had 12 in total. The 12 Shikigami （十二式神) found in Onmyōdō is exactly the same 12 Divine Heavenly Spirits (十二天將) found in esoteric Buddhism. This essentially paced Abe on a marble and gold pedestal as he was in the possession of the divine powers of the Kami themselves. He was without reproach in everything he did. Shikigami are invisible but they can be made visible by sealing them into small, folded and artfully cut Origami.
There are also shikigami that will appear as animals or birds. Using a complex ceremony, the Onmyōji connects the Shikigami to the spiritual force of their new master. Unlike a thought-form, a true Shikigami is a spirit servant bound to you by your own energy. Often, they agree to serve only if they are able to feed off of you in return. This can be dangerous if one is inexperienced in this practice. Because of this danger it was also outlawed.
The Seimei Shrine (晴明神社)
Located in Kyoto, Japan the shrine was built in 1007, just two years after Abe passed away. It was constructed at the site of his home, believing that his power would still emanate from this locality. The shrine is decorated with Shinto trappings but include his trademark pentagrams.
The restrictions have been lifted, and as of 2006 anyone may study Onmyōdō with exception to the forbidden magic. However, it is considered a New Religious Movement (NRM) as it is mainly one built from reconstruction.