The correct reading is いぜん. 已 (い) and 然 (ぜん) use their On'yomi, with 然 undergoing rendaku. 已's 'i' On'yomi is specific to this term. Wrong readings like やぜん or ひぜん misinterpret 已. 已然 refers to a state or event that has already occurred or is already established, often implying something that is 'as it is' or 'already done'.

正しい読みは「いぜん」です。已(い)と然(ぜん)の音読みが連濁しました。「やぜん」や「ひぜん」は已の誤読です。已然は、すでに起こった事柄や、そのままである状態を表す言葉で、完了の意味合いを持ちます。