Short answers, pulled from the story.
Hostilian, full name Gaius Valens Hostilianus Messius Quintus, was a Roman emperor who died around July 251. He served briefly as co-emperor after Trebonianus Gallus elevated him following the death of his father Decius at the Battle of Abritus in June 251.
Hostilian became co-emperor when Trebonianus Gallus elevated him to augustus after the Battle of Abritus in June 251. Gallus made the move to placate the public following the abrupt deaths of the previous emperors, Decius and Herennius Etruscus, who were Hostilian's father and brother.
The cause of Hostilian's death is disputed. Aurelius Victor and the Epitome de Caesaribus say he died of a plague, while the historian Zosimus claims Trebonianus Gallus had him killed. He died in or before August 251, most likely in July.
Some historians have identified Hostilian as the Roman general depicted in the Ludovisi Battle sarcophagus. This identification is considered unlikely because all of Hostilian's coins show him as a beardless young boy, inconsistent with the mature figure on the sarcophagus.
Hostilian's gold aurei fall into four types, all bearing his bust on the obverse. Their reverses depict Mars walking to the right, priestly implements, Mercury standing, and Roma seated holding Victoria.
After Hostilian's death, Trebonianus Gallus's son Volusianus was elevated to augustus and became the new co-emperor. This completed the transfer of power from the Decian dynasty to the Gallan one.