Short answers, pulled from the story.
Macrinus, born around 165 in Caesarea (modern Cherchell, Algeria), was Roman emperor from April 217 to June 218. He was the first emperor to come from the equestrian rather than senatorial class and the first emperor of Mauretanian descent. He was also the first emperor who never visited Rome during his entire reign.
Macrinus conspired to have Emperor Caracalla murdered on the 8th of April 217, reportedly to protect himself after a prophecy suggested he would one day depose the emperor. A soldier named Justin Martialis carried out the killing near the site of the battle of Carrhae. On the 11th of April, Macrinus proclaimed himself emperor without waiting for Senate approval, backed by the army.
Macrinus revalued the denarius, raising its silver purity from 50.78 percent to 57.85 percent and its weight from 1.66 grams to 1.82 grams. He also reduced the pay of new military recruits back to the rates set by Septimius Severus, while allowing soldiers already enlisted under Caracalla to keep their higher pay of 3,000 sesterces per year.
After an indecisive battle at Nisibis in 217, Macrinus paid a large indemnity to the Parthian ruler Artabanus IV in exchange for peace. He settled the Armenian conflict by returning the crown and looted property to Tiridates II, son of the imprisoned Khosrov I, releasing Tiridates' mother from prison, and restoring Armenia as a client kingdom of Rome.
Julia Maesa, aunt of Caracalla, engineered the revolt by persuading soldiers of the Legio III Gallica that her fourteen-year-old grandson Elagabalus was Caracalla's illegitimate son. The legion proclaimed Elagabalus emperor on the 16th of May 218. Macrinus was defeated at the Battle of Antioch on the 8th of June 218, near the village of Immae approximately 24 miles from Antioch, when he deserted the battlefield.
Damnatio memoriae was a Roman practice of erasing a condemned person from the historical record. After Macrinus' execution in Cappadocia, the Senate declared him and his son Diadumenianus hostes (enemies of the state), ordered their names struck from inscriptions and papyri and their portraits destroyed. The damnatio against Macrinus is among the earliest such sanctions the Senate enacted; many surviving marble busts of him show deliberate defacement.