Death-smoke is a mage-created fog that can be set, unmoving in one place. The touch of the fog is fatal. It is described as "a leprous blue-white, thick and oily, too murky to see into." (Winds of Fate)
Mornelithe Falconsbane used death-smoke to trap a herd of young dyheli in a pocket canyon. The spell was dispelled by Nyara, who had recently escaped her father. Unbeknownst to anyone else, it was all part of Falconsbane's plan to insinuate Nyara into the midst of Clan k'Sheyna and its allies.