Amaranthine gas

Amaranthine gas is a dangerous substance found in certain caves in the Subspace Emissary. When characters enter the purple mist, they quickly start taking damage at about 2% every half-second, depending on the difficulty. However, characters do not flinch while in the gas. Whenever amaranthine gas is present, there is always a switch nearby that, when hit, will temporarily clear the gas; the length of time the gas is deactivated depends on the difficulty and location.

Trivia

 * In some instances, if dashes through the gas wearing a Bunny Hood, he may not take damage.
 * The Poison Cloud in Super Smash Bros. Ultimate's spirit battles have a similar concept to amaranthine gas as they are both purple gaseous substances that deals damage to the player.
 * In addition, Piranha Plant's Poison Breath could be considered a miniature version of this, as they have the same effects with the Poison Breath being much smaller.
 * The gas is similar in appearance to the gas in Poisonous Pipeline from Donkey Kong Country 3: Dixie Kong's Double Trouble.