Shōki

Shōki, (Shouki, (瘴気, (しょうき,) lit. translation, "Swamp Gas." English Dub; "Miasma," ("Bad Air.")

Miasma is highly-corrosive and poisonous gaseous mist, a purple, smoke-like form, used by many demons as a means of both attack and/or defense. The Jaki of any Yōkai who can generate a miasma is also present in their miasma, like as in a unique signature. The more potent one's jaki, the more powerfully-corrosive their shōki/miasma becomes.

Naraku makes a widespread use of miasma, even using it as a means of delivery of his Jaki to infect people.

Trivia
The term Shoki in myth and the supernatural refers to ki/qi of a malign influence which is often considered to be avoided and dispelled, particularly with the arts of Feng Shui and Onmyodo. Due to Shinto's adaptable syncretization with other religions and beliefs, Shoki is also related with demons and the state of kegare due to its corrupting and foul nature. Shōki 瘴氣