In 177 Lucius was made coruler and heir to his father, the emperor Marcus Aurelius (reigned 161–180). Lucius joined Marcus in his campaign against invading German tribes along the Danube, but after the death of Marcus (March 180) he quickly came to terms with the Germans.

Soon after he became sole ruler, Lucius changed his name to Marcus Aurelius Commodus Antoninus. In 182 Commodus' sister Lucilla conspired with a group of senators to assassinate him. The plot failed, and Commodus retaliated by executing a number of leading senators. Thereafter, his rule became increasingly arbitrary and vicious. In 186 he had his chief minister executed in order to appease the army; three years later he allowed the minister's successor to be killed by a rioting crowd. Political influence then passed to the emperor's mistress and two advisers.

Meanwhile, Commodus was lapsing into insanity. He gave Rome a new name, Colonia Commodiana (Colony of Commodus), and imagined that he was the god Hercules, entering the arena to fight as a gladiator or to kill lions with bow and arrow. Finally, when Commodus announced that he would assume the consulship on Jan. 1, 193, dressed as a gladiator, the public became incensed. On Dec. 31, 192, his advisers had him strangled by a champion wrestler. A grateful Senate proclaimed as emperor the city prefect, Publius Helvius Pertinax, but the empire quickly slipped into civil war.