Shōki

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

Miasma is highly-corrosive and poisonus gasious mist, a purple, smoke-like form, used by many demons as a means of both attack and/or defence. The Jyaki of any Youkai who can generate a miasma is also present in their miasma, like as in a unique signature. The more potent one's jyaki, the more powerfully-corrosive their shōki/miasma becomes.

Naraku makes a wide-spread use of miasma, even using it as a means of delivery of his Jyaki to infect people.