Death Mist is a supernatural force, similar to the Mist but actually works backwards. It shrouds demigods and mortals from the presence of monsters. However, the Death Mist also turns the user into mist with a corpse-like complexion.
Akhlys, the owner and granter of the Death Mist, sets a trap for Percy Jackson and Annabeth Chase, by leading them to Nyx once they have the Death Mist. However, they escape, and use the Death Mist to conceal themselves from the army of monsters who guard the Doors of Death. Eventually the Death Mist wears off, revealing Percy and Annabeth to the monsters.