Spectre

A Spectre is a type of undead creature that is mentioned in the Mediochre Q. Seth series. In Black, White and Shades of Mediochre, Joseph mentions that even Spectres become visible if you force trillions of them into a small space and therefore it doesn't make sense that Dhampinella can detect seventeen trillion undead lifeforms in a single flat. The actual reason for the detection is the presence of undead bacteria.