Which term best describes a harmful or unpleasant emanation?

The term that best describes a harmful or unpleasant emanation is "effluvia." This word specifically refers to the outflow of vaporous substances, often with negative or foul connotations, such as noxious fumes or unpleasant odors. It captures the essence of something that is not only emanating but is also detrimental or unpleasant, aligning perfectly with the description provided in the question.

The other choices do not fit this definition. "Encomium" refers to a formal expression of praise, which is clearly not related to harmfulness or unpleasantness. "Exscind" means to cut out or remove, which does not pertain to emanations at all. Lastly, "epithet" refers to a descriptive phrase or term used to characterize someone or something, often in a laudatory or pejorative sense, but again does not convey the essence of something harmful or unpleasant emanating. Therefore, "effluvia" is the most appropriate choice given its specific meaning.