The civil war during the year of the Four Emperors was described by ancient historians as a troubling period . According to Tacitus , this instability was rooted in the fact that emperors could no longer rely on the perceived legitimacy of the imperial bloodline , as Nero and those before him could . Galba began his short reign with the execution of many allies of Nero and possible future enemies . One such notable enemy included Nymphidius Sabinus , who claimed to be the son of Emperor Caligula .