Characters under the effects of Heroes' Feast are immune to this spell's damage
Cloudkill is considered a "granted" condition not unlike that of a "granted" spell. Granted spells are issued to the caster after using the original, and can be can be recast without spending another spell slot as long as Concentration is not broken. This (10-turn) condition is not directly tied to the casters' repertoire of known spells (i.e., their spellbook icon in the game). Therefore, if a granted spell (or in this case, a granted condition) is used when multiclassing, subsequent re-uses (in this case, Cloudkill) use the spellcasting modifier of the most recently acquired spellcasting class rather than the spellcasting modifier that was used for the initial casting of Cloudkill. In the case of this condition, it may result in an unexpectedly low spell save DC.
Bugs
If upcast to level 6, Cloudkill creates an invisible cloud due to the surface lacking an attached particle effect.
NPCs will not automatically leave this cloud, as its surface ID is missing from character PathInfluence.