Caracalla, while traveling from Edessa to continue the war with Parthia, goes to visit a temple of Luna near the spot of the battle of Carrhae on April 8, 217, accompanied only by his personal bodyguard, which includes Macrinus.

The emperor’s escort gives him privacy to relieve himself at a roadside near Carrhae.

Events are not clear, but it is certain that Caracalla is assassinated, reportedly while urinating.

Caracalla's body is brought back from the temple by his bodyguards, along with the corpse of a fellow bodyguard.

The story as told by Macrinus is that the dead guard—Julius Martialis—had killed Caracalla with a single sword stroke.

Herodian says that Martialis' brother had been executed a few days earlier by Caracalla on an unproven charge; Cassius Dio, on the other hand, says that Martialis was resentful at not being promoted to the rank of centurion.

Macrinus, who (according to Herodian) was most probably responsible for having the emperor assassinated, has proclaimed himself emperor by April 11.

Macrinus also nominates his son Diadumenianus Caesar and successor and confers upon him the name "Antoninus", thus connecting him with the relatively stable reigns of the Antonine emperors of the second century.