Rabadash is the eldest of the eighteen sons of the Tisroc and crown prince of Calormen. He is self-important and is a cruel, wild man who is not accustomed to having his wishes crossed.
In 1014 NT, he and a group of Calormens traveled to Narnia as ambassadors to see if Queen Susan's beauty was as great as reported. Rabadash wishes to marry Susan, but his request for her hand may only be for sinister intentions. He invites the Narnians to visit him in his own country, and hosts King Edmund and Queen Susan and some of their court at his father's palace in Tashbaan.
When it is insinuated that Queen Susan intends to reject his offer of marriage, Rabadash plans to have them held by force until she agrees. But the Narnians slip away without his knowledge, and Rabadash is furious. He immediately concocts a plan to ride across the desert, take Archenland by surprise and then ride down into Narnia, killing the kings and capturing Susan. Pleased with a plan that will give him control of two small countries he's wanted for a long time, the Tisroc agrees to the plan and Rabadash sets off as soon as he can gather men and horses.
Unfortuantly, his plan was overheard by Aravis, and when Rabadash arrived at Anvard he found the king warned of his attack and prepared for seige. Despite losing the crucial element of surprise, Rabadash attacks anyway but is utterly defeated when the Narnians arrive to Archenland's aid. Rabadash is defeated in combat by King Edmund, and manages to get himself stuck on a hook on the castle, to his utter humiliation and disgrace.
Rabadash is taken into Anvard as a prisioner, and is brought before the King and his Lords to have his fate decided. Rabadash rejects any offers of mercy, and treats the Narnians and Archenlanders with contempt. When Aslan suddenly appears, Rabadash ignores his warnings and continues to shout out curses and insults, and finds himself suddenly turned into a donkey.
He is returned to Calormen in disgrace, and is freed of his donkey form by Tash at his temple, but was told by Aslan that if he ventured ten miles from the temple, he would turn back into a doneky permanantly.
He is known the rest of his days as Rabadash the Peacemaker, although everyone calls him Rabadash the Ridiculous behind his back. It becomes a Calormen insult to be known as a "second Rabadash", and he lives out the rest of his days in humiliation.