Poison fog

Poison fog, also known as ash, is an obstacle first found in New Super Mario Bros. Wii, normally seen as an enormous red-and-black cloud. It normally moves to the right very slowly. It is likely based on.

New Super Mario Bros. Wii
In New Super Mario Bros. Wii, ash is solely found in World 8-1. It kills the player instantly upon being touched. However, neither the volcanic debris nor the Buzzy Beetles are affected by it.

New Super Mario Bros. U / New Super Luigi U
Ash reappears in New Super Mario Bros. U in Magma-River Cruise, where it behaves the same way. It also appears in the challenge Mole to Goal Returns and in New Super Luigi U's Magmaw River Cruise.

Super Mario 3D World / Super Mario 3D World + Bowser's Fury
Poison fog appears in Super Mario 3D World and Super Mario 3D World + Bowser's Fury, where instead of moving right, it moves down. It also appears to generate electricity in this game. Like in past appearances, it will make the player lose a life if they touch it, regardless of whether or not they have a power-up. It appears only in Trick Trap Tower, where once it reaches the bottom, it sinks into the lava, causing it to rise. It appears when the Key Coin at the top of the tower is collected.