Duke Mephitis of the Vapour was a Chthonian demon serving under Zaros. He led the eighth of the twelve demonic legions given to Zaros by Hostilius. Alongside Duke Orcus and Duke Sucellus, he remained one of the only Chthonian dukes who served as the leaders of a legion by the time Damaged letter was written.
- Mephitis is the name of a minor goddess of poisonous gases in Roman mythology, from whose name the English word "mephitic" (meaning foul-smelling vapour) comes from. This is reflected by the duke's title.