Miraz is the second son of Caspian the Eighth and Lord Protector of Narnia. He conspires to have his brother, King Caspian the Ninth murdered, and then quietly weeds out all those who opposes him.. Once this is accomplished, he crowns himself King of Narnia.
Miraz is very controlling, and does not allow his subjects to speak about the Old Narnia whom his ancestors had conquered, because he believes, or likes to believe, that they are nothing more than fairy tells. He even banishes his own nephews nurse after learning that she had been teaching him stories about Narnia before the Telmarines.
With no male heir of his own, Miraz is content to have his brother's son Caspian be his heir, and schools him in the art of kingship. However, after his wife Prunaprismia gives birth to a son in 2303 NT, he no longer wants Caspian around and plans to have him killed. However, Caspian is warned of the impending doom and escapes, joining up with the Old Narnians who are in hiding, and makes plans to resore the kingdom to those to whom it rightfully belongs.
Miraz pursues Caspian and wages war with him and the new-found Narnians. He is successful and outnumbers Caspian considerably, but Caspian summons the old Kings and Queens from the past, and the High King Peter issues a challenge of single combat to Miraz, so that the war may end with little bloodshed.
Miraz has no intention of accepting such a risky challenge, but is pursuaded to accept by several of his Lords, who unbeknownst to him are secretly plotting to kill the king.
During the duel, Miraz fights well but is eventually overcome by Peter's skill and youth. Near the end, he trips over a tussock of grass and falls to the ground helpless. While Peter steps aside to allow him to regain his footing, Miraz's Lord Glozelle runs up to him under the pretense of helping him up, and stabs him fatally in the back.