Cloudkill creates a toxic cloud which will kill or do poison damage to anyone caught in it. Spells or items that boost Save vs. Breath will not help to defend against this spell.
This spell generates a billowing cloud of ghastly, yellowish green vapors that is so toxic as to slay any creature with 4 or fewer Hit Dice, and causes creatures with 5 to 6 Hit Dice to roll Saving Throws vs. Poison with a -4 penalty or be slain. Holding one's breath has no effect on the lethality of the spell. Those above 6th level (or 6 Hit Dice) must leave the cloud immediately or suffer 1d10 points of poison damage each round while in the area of effect.