In this sentence, "es" is purely decorative, it's only a placeholder that makes the mood a bit more "passivish". You rephrase the same sentence without "es": Gewiss, ihn hat nie jemand zu entdecken vermocht, ... So "jemand" is the subject (nominative) and "ihn" is the direct object. "Es" is a sort of additional neutral subject, if you want. More details: http://www.canoo.net/services/OnlineGrammar/InflectionRules/FRegeln-P/Pron-es.html?lang=en#Anchor-es-11481