The progression of the epithalamus over time from its initial formation until its mature state. The epithalamus is the small dorsomedial area of the thalamus including the habenular nuclei and associated fiber bundles, the pineal body, and the epithelial roof of the third ventricle. [ GOC:dgh http://purl.obolibrary.org/obo/go/references/0000021 https://www.worldcat.org/search?q=bn%3A0838580343 GOC:jid GOC:cls GOC:dph ]