Death Mist (PJO/HOO/TOA)

Death Mist is a supernatural force, similar to the Mist but actually works backwards. It shrouds demigods and mortals from the prescence of monsters. However, the Deat Mist also turnds the user into mist with a corpse-like complexion.

The House of Hades
Akhlys, the owner and granter of the Death Mist, sets a trap for Percy Jackson and Annabeth Chase, bu 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 with the emergence of Tartarus himself personified, revealing Percy and Annabeth to the monsters, including Hyperion and Krios.

Trivia

 * According to Akhlys, the Death Mist represents Tartarus' own breathing.