Henry III of England

Henry III (1 October 1207 – 16 November 1272), also known as Henry of Winchester, was King of England and Lord of Ireland from 1216 to his death.

In 1215, a baronial uprising led by the Assassin Robert Fitzwalter was initiated against Henry's father, the Templar-influnced King John. Operating independently from his Brotherhood, the Assassin William of Cassingham supported John and Henry, seeing this avenue as the lesser of two evils.

The year after, John died, leaving the nine-year-old Henry the King of England. By this time, much of the country was under the control of the barons' ally, Prince Louis of France. However, the barons soon rallied to support Henry, and the prince was expelled.

Reference

 * Assassin's Creed: Memories