Cloudkill

This spell generates a bank of fog, its vapors yellowish green and poisonous. These vapors automatically kill any living creature with 3 or fewer HD (no save). A living creature with 4 to 6 HD is slain unless it succeeds at a Fortitude save (in which case it takes 1d4 points of Constitution damage each round while in the cloud). A living creature with more than 6 HD takes 1d4 points of Constitution damage each round while in the cloud (a successful Fortitude save halves this damage). Holding one's breath doesn't help, but creatures immune to poison are unaffected by the spell. The cloudkill moves away from you at 10 feet per round in the direction you are facing, rolling along the surface of the ground.