On January 10, 69 CE, Emperor Galba chose a man to become his deputy and heir. One of Galba’s advisors had led Marcus Otho to expect to be appointed to this office, but instead Galba chose one Piso Licinianus. This unexpected choice led Otho to conspire to assassinate both of them and seize power.
On January 15 Otho struck. Galba and Piso were being carried on litters through the street when they were accosted by a large company of renegade Praetorians in Otho’s employ. The Praetorians were supposed to be the personal bodyguard of the emperor, but now they intended his death.
Of all the soldiers present, only Sempronius Densus, who had been assigned by Galba to guard Piso, stood firm, while his colleagues either joined in the murder or melted away. While Piso fled to seek a safe hiding place, Sempronius bought him time to escape, first remonstrating with the assassins and then fighting them to the death.
At this point sources differ slightly. According to Plutarch, Sempronius gave his life defending both Galba and Piso:
No man resisted or offered to stand up in his defence, save one only, a centurion, Sempronius Densus, the single man among so many thousands that the sun beheld that day act worthily of the Roman empire, who, though he had never received any favour from Galba, yet out of bravery and allegiance endeavoured to defend the litter.
First, lifting up his switch of vine, with which the centurions correct the soldiers when disorderly, he called aloud to the aggressors, charging them not to touch their emperor. And when they came upon him hand-to-hand, he drew his sword, and made a defence for a long time, until at last he was cut under the knees and brought to the ground.
After Sempronius finally fell, the assassins surrounded Galba and killed him.