Flaming Altar
Carus was killed by lightning while on campaign in Persia. A person struck by lightning was considered by the ancients to have provoked the extreme wrath of the gods and an ancient prophecy stated that the River Tigris was to be the Eastern boundary of the Roman Empire. Perhaps the flaming altar was to indicate to the gods that the message had been received as they retreated from Persia.
Milne 4733 Carus
Curtis(1990) p. 197 coin 384v