The toxin is an incredibly deadly gas.
.source
The toxin is a byproduct of the Hellfire storms of the Wild Hunt, found throughout the entire planet, as high up as the exosphere and diffused in all bodies of water, with no depth limit.
.properties
The toxin is pink, iridescent, nonflammable, odorless, and extremely dangerous to all forms of life. Once inhaled, it will cause choking, coughing fits, and death in less than a minute. Survival is possible if treated fast enough, although it will leave behind extreme internal damage and scarring, including neuronal damage. Organisms native to the FLIPSIDE are completely immune to it. There is no easy or reliable way to destroy or neutralise it.