Short answers, pulled from the story.
Severus Alexander became emperor at around 13 years old, making him the youngest sole emperor of the united Roman Empire. The Roman army hailed him as emperor on the 13th of March 222, and the Senate ratified his acclamation the following day.
Severus Alexander was assassinated in early March 235 at Moguntiacum, the city now called Mainz, in a mutiny of the Legio XXII Primigenia. He was killed inside a tent during a meeting with his generals; his mother Julia Mamaea was killed in the same attack.
His soldiers considered him dishonorable and unfit to command after he attempted to buy peace with Germanic tribes rather than fight them. Combined with poor results in the Persian war and Alexander's dependence on his mother's advice, the army transferred its loyalty to Maximinus, a soldier from Thrace with a stronger military reputation.
The Crisis of the Third Century was a period of nearly fifty years of civil war, foreign invasion, and monetary collapse that brought the Roman Empire to the brink of ruin. Alexander's death in 235 is the epoch event that opened this period; after his assassination, his economic policies were discarded and the Roman currency was devalued.
Julia Mamaea was Alexander's mother and his primary political adviser throughout his reign. She arranged the marriage that brought Orbiana into the imperial family and then arranged Orbiana's exile, and it was her advice to bribe the Germanic tribes rather than fight them that contributed to the loss of military support for Alexander.
The Portland Vase is a cameo glass vessel dated to around the reign of Augustus that was allegedly found in the 16th century at Monte del Grano, the mausoleum traditionally believed to hold Alexander's remains. An early account claimed it contained his ashes, though this is disputed. The vase was sold to the Duchess of Portland in 1784 and was smashed at the British Museum in 1845; it has since been reconstructed three times.