Answer
The answer is Emperor. By marrying Empress Matilda, daughter of Henry I, Geoffrey Plantagenet, Count of Anjou, became the sole legitimate heir to the English throne. Matilda's claim was contested by King Stephen, leading to a prolonged civil war. Upon Stephen's death in 1154, Henry II, Geoffrey and Matilda's son, ascended to the throne as the first Angevin king of England, initiating the Plantagenet dynasty.