Describe the crisis that afflicted the Roman empire after the Pax Romana ended?

It was crisis of political instability. There were conspiracies against emperor Commodus, who was described as a coward and more interested in sport than politics, followed by struggles between five men who wanted to usurp the title of emperor. The crisis saw the end of the Nervan-Antonine dynasty and the beginning of the Severan dynasty. The establishment of the new dynasty ended this period of political instability.

On the death Of Marcus Aurelius in 180, his son Commodus succeeded him. His sister Lucilla arranged a conspiracy against him. The assassination was bungled. Commodus' treasurer was murdered in a second conspiracy. There was unrest in Gaul and Germany caused by army deserters and an army revolt in Brittany. A plot by one of the deserters to assassinate Commodus was betrayed. Another plot was uncovered and foiled in the same year. Cleander, the new treasurer, deposed the praetorian prefect (the commander of the praetorian guard, the bodyguards of the emperor) and sold public offices to strengthen his political position with the intent of becoming more powerful than the emperor. However, he was executed following a riot against him. After this, Commodus decided to take charge of his government, but he was murdered in the following year.

This was followed by the year of the five emperors, where five usurpers claimed the title of emperor. Pertinax was proclaimed emperor by the praetorian guards, but was murdered by them, apparently because he gave them only half of the pay off for making him emperor. Then Didius Julianus outbid a rival in paying off the soldiers and was proclaimed emperor. A demonstration broke out and people called for Prescennius Niger, governor of Syria, to become emperor. He was proclaimed emperor by the eastern legions. However, Septimius Severus marched on Rome and took over. He was challenged by Clodius Albinus who was proclaimed emperor by the legions of Britannia and Hispania. He crossed into Gaul with his troop and defeated Septimius Severus' general in Gaul. He established his headquarters in Lyon, but was defeated there in a large battle by Septimius Severus, who then managed to consolidate his power and start a new dynasty.