Seimei-sama no Ossharu Koto ni wa Heian Inyo Shinryoroku

Watanabe Harunobu strives to become a full-fledged samurai, despite harboring a certain secret. One day, feeling unwell, Harunobu meets the mysterious onmyoji Abe no Seimei, who is able to pinpoint the cause of his condition. Furthermore, in front of Harunobu's eyes, Seimei extends the lifespan of an aristocrat who had stopped breathing after falling into a pond. When Harunobu asks Seimei about his mysterious power, Seimei reveals that it is "medical arts." Seimei then uncovers Harunobu's secret and pressures him to become his assistant...

This is a Heian medical fantasy in which an onmyoji and his troubled assistant exorcise illnesses.

(Source: Natalie, translated)