Buff
Poison Cloud
The Poison Cloud buff blocks damage from Poison debuffs. Also, enemies can only land weak hits on Champions under a Poison Cloud buff.
If a Champion receives an HP Burn debuff before a Poison Cloud buff is applied, then the effects of Poison Cloud won't work, but the buff itself will not be removed. It's not affected by the Spread Buff and Increase Buff Duration effects.
What the game's own status effects screen says
In the game data this effect appears as PoisonCloud.
Champions that apply it
2 champions have a skill that applies Poison Cloud, according to the effect lines in the game data. This lists who applies it, not how reliably: chance, duration and resistance are on each champion's page.
The description is the game's own text. It has not been tested in battle here, and the game's wording can be loose (for example about numbers that differ between versions of an effect). See Data quality for what has and has not been checked. Found a mistake? Use "Report a problem" at the bottom of the page.