Answer
The answer is Yazid I, the son of Muawiyah I, founder of the Umayyad dynasty. Muawiyah I successfully contested the caliphate after the first Muslim civil war (Fitna) and established the Umayyad Caliphate, marking a shift from an elective system of succession to a hereditary one. This transition had a profound impact on the political structure of the early Islamic empire, consolidating power within the Umayyad family for several decades.