What happened April 217 AD

Caracalla was assassinated while en route to a campaign against the Parthians by the Praetorian Guard.

On April 8, 217, Caracalla was assassinated traveling to Carrhae. Three days later, Macrinus was declared Augustus. Diadumenian was the son of Macrinus, born in 208. He was given the title Caesar in 217, when his father became augustus, and raised to co-Augustus the following year.