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Onmyōdō (陰陽道, also In'yōdō, lit. ‘The Way of Yin and Yang’) is a traditional Japanese esoteric cosmology, a mixture of natural science and occultism. It is based on the Chinese philosophies of Wu Xing and Yin and yang, introduced into Japan at the beginning of the 6th century. It was accepted as a practical system of divination. These practices were influenced further by Taoism, Buddhism and Shintoism, and evolved into the system of onmyōdō around the late 7th century. Onmyōdō was under the control of the imperial government, and later its courtiers, the Tsuchimikado family until the middle of the 19th century, at which point it became prohibited as superstition.
In the 5th and 6th centuries, the principles of yin-yang and the Five Elements were transmitted to Japan from China along with Buddhism and Confucianism. Yin-yang and the Five Elements, as well as the divisions of learning to which they were linked – astronomy, calendar-making, the reckoning of time, divination, and studies based on observation of nature – were amalgamated into fortune telling. This process of judging auspicious or harmful signs present in the natural world was accepted into Japanese society as a technique for predicting good or bad fortune in the human world. Such techniques were known mostly by Buddhist monks from mainland Asia who were knowledgeable in reading and writing Chinese. Over time demand from members of the imperial court who believed that onmyōdō divination would be helpful in decision-making made it necessary for the laity to perform the art, and onmyōji began to appear around the middle of the 7th century.
With the implementation of the ritsuryo system law codes in the 7th and 8th centuries, yin-yang techniques were put under the jurisdiction of the Bureau of Onmyō (陰陽寮 Onmyō-ryō ) in the Nakatsukasa-shō of the Imperial bureaucracy. The Bureau of Onmyō was responsible for overseeing the divinations of Onmyōdō, astrological observations, and the creation of calendars. Also, by law the Buddhist clergy was forbidden to practice astrology and fortune-telling; hence, government-controlled onmyōji came to monopolize the practice.
From the Heian period onward, as the ritsuryo system relaxed and the Fujiwara family rose to power, the society of the Imperial court took on a more formal shape, and adherence to rituals to placate the souls of the dead (御霊信仰 Goryō Shinkō ) to combat the creation of vengeful ghosts (怨霊 onryō ) burgeoned. Because onmyōji displayed methods that were believed to avert disasters with their skills of divination and magic, the practice afforded onmyōji influence over the personal lives of the Emperor and the nobility of the court. Consequently, popular awareness of onmyōdō gradually spread from court society to Japanese society as a whole, strengthening its development into a characteristically Japanese art.
Onmyōdō merged with other beliefs and occultism, and evolved from imported Chinese thought into a syncretism found only in Japan. Japanese onmyōdō took in elements from Taoism (道教), which was transmitted to Japan at the same time as onmyōdō, including magical elements such as katatagae, monoimi, henbai, and ceremonies to Taoistic gods such as the Taizan Fukunsai. Elements of feng shui and the medical art of jukondō were incorporated as well, and as onmyōdō and Japanese Shinto mutually influenced each other, onmyōdō grew more distinctive. From the end of the 8th century onward, it was influenced by the magical elements of esoteric Buddhism and the Indian-derived astrology (Sukuyōdō) that were transmitted with it.
During the Heian period the nobility organized their lives around practices recommended by onmyōji. The practice of "lucky and unlucky directions" provides an example. Depending on the season, time of day, and other circumstances, a particular direction might be bad luck for an individual. If one's house was located in that direction, such an individual was advised not to go back directly to his house but had to "change direction" (katatagae), by going in a different direction and lodging there. Such a person would not dare to go in the forbidden direction, but stayed where they were, even if that resulted in absence from the court, or passing up invitations from influential people.
In the 10th century Kamo no Tadayuki and his son Kamo no Yasunori made great advancements in onmyōdō, astronomy and calendar science, and from among their students emerged Abe no Seimei, who displayed superior skills in the divining arts of onmyōdō, by which he gained an uncommon amount of trust from the court society. Tadayuki and Yasunori passed on their skills in astronomy to Seimei while their advances in calendar-making went to Yasunori's son. From the end of the Heian period into the Middle Ages, astronomy and calendar science were completely subsumed into onmyōdō, and the Abe and Kamo families came to dominate the art.
Onmyōji (陰陽師, also In'yōji) was one of the classifications of civil servants belonging to the Bureau of Onmyō in ancient Japan's ritsuryo system. People with this title were professional practitioners of onmyōdō.
Onmyōji were specialists in magic and divination. Their court responsibilities ranged from tasks such as keeping track of the calendar, to mystical duties such as divination and protection of the capital from evil spirits. They could divine auspicious or harmful influences in the earth, and were instrumental in the moving of capitals. It is said that an onmyōji could also summon and control shikigami.
Onmyōji had political clout during the Heian period, but in later times when the imperial court fell into decline, their state patronage was lost completely. In modern day Japan onmyōji are defined as a type of Shinto priest, and although there are many that claim to be mediums and spiritualists, the onmyōji continues to be a hallmark occult figure.
- Kinugyokutoshū (金烏玉兎集/三国相伝陰陽輨轄簠簋内伝金烏玉兎集, "the collection book of the moon and the jade rabbit")
- Senji Ryakketsu (占事略决, "the summary to judgements of divinations") by Abe no Seimei
- The 1985 historical fantasy novel Teito Monogatari by Hiroshi Aramata is widely credited with starting the contemporary boom of interest in onmyōdō mysticism in Japan.  The novel details a fictional account of a battle between two onmyoji factions in Tokyo at the beginning of the 20th century. The protagonist of the novel, Yasunori Kato, is a heretical onmyoji who wants to cripple the Japanese Empire and he must contend with the Tsuchimikado Family; the leader of which is Yasumasa Hirai, who is the direct descendant of Abe no Seimei.
- The film Onmyōji is a fictitious account of Abe no Seimei based on the novel series of the same name (which also inspired a manga adaptation, also of the same name).
- Lin Koujo from Ghost Hunt is considered by most to be an onmyōji. He frequently exorcises spirits, summons spirits, controls shikigami, and possesses much expertise in the subject of eastern occultism in general.
- Subaru Sumeragi, from CLAMP's Tokyo Babylon and X, is an onmyōji who protects modern Japan from curses and vengeful spirits. Protecting Japan from the shadows is the onmyouji slash assassin Seishirō Sakurazuka, Subaru's dark counterpart.
- In Shaman King, onmyōji are a type of high-level shamans. Characters include Yoh's grandfather Yohmei Asakura, and Hao. Also, there is a book named Chō Senji Ryakketsu written by Hao, known to contain powerful shamanic techniques, some of which are based on Wu Xing principles.
- The character Motoharu Tsuchimikado from the Toaru Majutsu no Index light novel series is a former onmyōji and a descendent of Abe no Seimei
- Abe no Yasuaki from the game and anime Harukanaru Toki no Naka de is an onmyōji and a disciple of Abe no Seimei.
- Ken Akamatsu's Negima!: Magister Negi Magi features a sophisticated, secretive culture of onmyōji.
- The protagonist of Gainax's Magical Shopping Arcade Abenobashi is drawn to a parallel world where he learns the art of onmyōdō from Abe no Seimei.
- Rental Magica's Nekoyashiki is a onmyōji who usually calls upon cat shikigami.
- The anime and manga Onmyō Taisenki revolves almost completely around onmyōdō.
- Ask Dr. Rin! features an onmyōji named Tokiwa Takashi.
- Keiya Tenpouin is an onmyouji from the PlayStation game Evil Zone.
- Shōnen Onmyōji is a 26 episode, anime series that chronicles the life of Abe no Masahiro, as he trains to become a better onmyouji than his prodigiously famous grandfather, Abe no Seimi.
- Nakahito Kagura, the protagonist of Steel Angel Kurumi, belongs to a family on onmyou mystics.
- The PlayStation game Final Fantasy Tactics includes a Job called Onmyoji (translated as "Oracle" for the English release of the game). Onmyoji can use "Yin-Yang Magic" to inflict status abnormalities such as blindness, paralysis and sleep on their enemies.
- The series Nura: Rise of the Yokai Clan features Onmyōji working to defeat Abe no Seimei.
- One of the cards in Magic: The Gathering is called Goryo's Vengeance http://magiccards.info/scans/en/bok/67.jpg
- Hazuki (and/or Hazuki's mom) [In "Tsukuyomi's moon phase" anime] can summon a shikigami (Kouheis grandpa's sister made the shikigami)
- The main characters of the survival horror game Kuon are all onmyouji, and great part of the magic, seals and puzzles in the game are based on onmyōdō.
- Onmyōdō is the name of a footwear company.1
- The onmyōji Abe no Seimei and Kamo no Yasunori were the inspiration for the Tao Troopers organization members Abe and Kamo in the PlayStation 2 game Ōkami.
- The manga Tokyo Ravens is based on the Onmyōdō and Onmyōji theme, also featuring the main character as part of the Tsuchimikado family and a descendant of Abe no Seimei.
- Kanna, a playable Magician-type character class in the Japanese and Global versions of the MMORPG MapleStory, is described as an onmyōji in a side-story part of the game world set in the Sengoku period of Japan.
- Gintama has two Onmyouji clan, with characters named after legendary people such as Seimei.
- In Natsume's Book of Friends, Taki's family used to be Onmyōji but currently can no longer practice. Taki's grandfather collected many books and records about various yokai, spells and barriers which Taki has now inherited.
- In Neil Gaiman's The_Dream_Hunters, an evil onmyōji plans to kill a monk a Kitsune has fallen in love with.
- Kazuhiko, Komatsu. "Seimei jinja" 28-61