Cloudkill

This spell calls into being a poisonous cloud of roiling vapour, heavier than air, which moves along the ground (or into holes) at a movement rate of 10 ft/ round. The eldritch poison of the cloud is so virulent that it kills creatures of fewer than 4+1 HD instantly, with no saving throw. Creatures with 4+1 HD up to 5+1 HD are allowed a saving throw at –4, and creatures of 5+2 up to 6 HD must make a normal saving throw. Failure on the saving throw means that the victim dies. The spell is affected normally by winds, although it magically avoids rolling back over the caster under any circumstances. It is dispersed in 4 rounds by a strong wind, and by thick growths of vegetation if it rolls through these for a distance of 20 ft.