Gas grants Mustard the ability to generate a poisonous, sleep-inducing gas from his body.
The user is capable of producing a large amount of purple-colored gas, enough to completely fill The Beast's Forest. Anyone who inhales the gas directly will immediately fall asleep and remain that way until the toxin is treated. 
The presence and movement speed of anyone within the gas can be sensed by the user. The gas' concentration is highest at its center and loses potency in lesser affected areas away from the user.
The gas can be filtered using any sort of gas mask, and lighter concentrations of it aren't as effective. The user is also incapable of manipulating the gas beyond its natural flow, meaning they cannot use their Quirk in combat. If the user loses consciousness, the gas dissipates completely.
Mustard is capable of flooding an entire forest with poison gas. He stays away from his primary targets, best serving as support for ambushing them instead. By keeping his distance and reading the flow of the gas, Mustard prevents himself from being taken by surprise.
The gas is very potent and takes effect immediately, and Mustard can spread the gas quickly to take out numerous people at once. However, anyone using a breathing apparatus can avoid the effects of his Quirk and challenge him in combat. Mustard carries a handgun to compensate for this weakness. He also wears a gas mask, implying that he's not immune to the effects of his own Quirk.
- This Quirk works similarly to Somnambulist. Unlike Gas, however, Somnambulist doesn’t have toxic properties, has no direct effect on the user, and isn’t potent enough to put someone into deep sleep.