To instruct and edify: Onmyouji is set in the 10th century. Tokugawa is the family name of the shoguns who ruled in the Edo period from the early 17th through late 19th centuries, while keeping the emperor a powerless and impoverished ceremonial figurehead in Kyoto. Tokugawa mikado? You might as well talk about the the Macarthur Emperor who waged war in 1252.

Daimyou are the regional clan heads in the Edo period, mostly descended from warriors who supported Tokugawa Ieyasu. What you see in Onmyouji are Kyoto nobility, mostly descended from younger brothers of earlier Emperors. However powerful the daimyou got, the attitude of the court nobles is that they were uncouth barbarians.