Epiphenomenalism is the concept which describes the sudden appearance of an emergent property (a new kind of property) which is unique in its property and character which appears
sui generis. If we assume that mental properties are genuinly emergent
sui generis properties, then given the mereological hierarchy and its top/down causation, the emergent mental properties presents at least two problems for naturalism. First, for those naturalists who accept a causal criterion of existence, emergent mental properties are epiphenomenal and thus do not esist in their mind. One must either accept phenomenal consciousness, which construes emergent mental properties along familiar lines as what-it-is-like to be such and such and rejects causal closure
or one retains causal closure and rejects phenomenal consciousness exactly because it is epiphenomal. That is to say with a metaphysical naturalist or physicalist that quality which is epiphenomenal cannot be reduced to a basal causation in the merelogical hierarchy. It cannot be reduced to any other that itself. It is not the stuff of this universe and is not compelled by the physical laws of the universe. Mental properties are emergent in the sense that they are genuinely new
kinds of properties.
I hope that helps.