Mist

Background
The Mist is a phenomenon which occurred in Bridgeville, Maine, USA in 2019. Not much is currently known about it, but it seems to be related to the urban legend of The Black Spring, which tells of nature, usually in the form of animals, kill off several members of the population.

Source
The source of the Mist is currently unknown, though there is some implication that it is a result of something to do with the Arrowhead Base, as several military members were randomly in town when it rolled in, and two of them killed themselves when the base could not be reached via radio.

Methods of Death
The Mist provides several different forms of death, though does not seem to be toxic in and of itself. At various times it is shown sending swarms of bugs to eat people, multiply within them, and sometimes even mutate them into humanoid insect creatures themselves.

There is also a large, dark shadow like monster which feels on people by sucking their life away. The Mist seems to cause hallucinations to draw its prey in. Mia Lambert saw her dead mother who tried to encourage her to commit suicide, as she has tried in the past. Nathalie Raven also saw her deceased husband holding her miscarried infant. She attempts to breastfeed the child but it ends up sucking the life force out of her as well.

The Mist causes fear and chaos, as it forces survivors to be trapped in doors to avoid exposure. This causes rationing of food and morale to become dangerously low as people begin to fight, loot, and kill one another to survive.