Short answers, pulled from the story.
A barracks emperor, also called a soldier emperor, was a Roman ruler who seized power through command of an army rather than through senatorial approval or hereditary succession. The term most specifically applies to the roughly fourteen men who held the throne between 235 and 284 AD during the Crisis of the Third Century.
The Crisis of the Third Century began in 235 AD with the assassination of Severus Alexander. It ended in 284 AD when Diocletian, a cavalry commander and the last of the barracks emperors, seized power and instituted reforms that stabilized the empire.
Maximinus Thrax was the first barracks emperor, reigning from February or March 235 to March or April 238. He had begun his career as an ordinary enlisted soldier and came from the outlying parts of the empire, lacking any distinguished family name or statesman's record.
There were approximately fourteen barracks emperors in the 33-year period from 235 to 284 AD, producing an average reign of a little over two years each. Many were killed by their own soldiers, died in battle, or were deposed by rivals.
Barracks emperors drained the Roman treasury by constantly paying soldiers to maintain loyalty, causing public works and infrastructure to fall into ruin. To cover military costs, emperors often seized private property, which damaged the economy and drove inflation upward across the empire.
The Aurelian Walls were built as a direct consequence of barracks emperors abandoning border defenses to march on Rome. When frontier commanders turned their armies inward to seize power, they left the borders unguarded, which led to Germanic incursions into Roman territory in the 260s.