Late antiquity has not been especially well-served by the novelist. I have a fondness for Manfredi's The Last Legion, despite the film and the dreadful English translation. Probably the best evocation of the collapse of the Western Roman Empire remains Iain Pears' The Dream of Scipio.
It is now worth flagging up The Boy Orestes a new novella by Gareth Power. It is the first of a series that wil continue until the resignation of Romulus Augustulus. The Boy Orestes is set in Pannonia in AD433 as Orestes, better known as the father of Romulus Augustulus, turns 16. Researched in some detail, characters of the period - Aetius and Avitus to name just two - make appearances. Worth a read!