Magnus was a Roman usurper

Roman usurper

Usurpers are individuals or groups of individuals who obtain and maintain the power or rights of another by force and without legal authority. Usurpation was endemic during roman imperial era, especially from the crisis of the third century onwards, when political instability became the rule.The...

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Magnus was a senator

Roman Senate

The Senate of the Roman Republic was a political institution in the ancient Roman Republic, however, it was not an elected body, but one whose members were appointed by the consuls, and later by the censors. After a magistrate served his term in office, it usually was followed with automatic...

 of consular rank

Consul

Consul was the highest elected office of the Roman Republic and an appointive office under the Empire. The title was also used in other city states and also revived in modern states, notably in the First French Republic...

. After the death of Emperor Alexander Severus

Alexander Severus

Severus Alexander was Roman Emperor from 222 to 235. Alexander was the last emperor of the Severan dynasty. He succeeded his cousin Elagabalus upon the latter's assassination in 222, and was ultimately assassinated himself, marking the epoch event for the Crisis of the Third Century — nearly fifty...

 there was much ill-feeling in the Senate about the elevation of Maximinus Thrax

Maximinus Thrax

Maximinus Thrax , also known as Maximinus I, was Roman Emperor from 235 to 238.Maximinus is described by several ancient sources, though none are contemporary except Herodian's Roman History. Maximinus was the first emperor never to set foot in Rome...

 to the throne. A group of officers and senators under the leadership of Magnus plotted to overthrow Maximinus. Their plan was to have Roman soldiers destroy the bridge over the river Rhine, after Maximinus had brought the army across during his campaign against the Germans

Germans

The Germans are a Germanic ethnic group native to Central Europe. The English term Germans has referred to the German-speaking population of the Holy Roman Empire since the Late Middle Ages....

. Maximinus would be left stranded on the north bank of the Rhine, at the mercy of the Germans.
Before it was realized, the plan was discovered and all conspirators executed.