Rosemary Sutcliff 
Mark of the Horse Lord 

'Take my place, Phaedrus, and with it, take my vengeance . . .’Phaedrus the gladiator wins his freedom after years of bloody battles in the arena. Soon he finds himself riding north towards the wilds of Caledonia on a strange mission. He is to assume the identity of Midir, Lord of the Horse People, to seek vengeance against the treacherous Liadhan, who has usurped the throne.Ahead of him lies more adventure and more danger than he had ever known in the arena . . .