Who conquered the Roman Empire?

Technically, nobody burst Rome into flames and started their own empire. Consuls who came far after Alexander the great, Julius Caesar and Octavian Augustus were part of foolish dynasties (Tarquins, Caligula, Nero, etc.), and this created disorder in Europe. By the time the Crusades and Alaric came around in the years of castles and knights and serfs, the Roman Empire was dead from warfare.