Elagabalus
The first sun god consistently termed invictus was the provincial Syrian god Elagabalus. According to the Historia Augusta, the teenaged Severan heir adopted the name of his deity and brought his cult image from Emesa to Rome. Once installed as emperor, he neglected Rome's traditional State deities and promoted his own as Rome's most powerful deity. This ended with his murder in 222.
The Historia Augusta refers to the deity Elagabalus as "also called Jupiter and Sol" (fuit autem Heliogabali vel Iovis vel Solis).
This has been seen as an abortive attempt to impose the Syrian sun god on Rome; but because it is now clear that the Roman cult of Sol remained firmly established in Rome throughout the Roman period, this Syrian Sol Elagabalus has become no more relevant to our understanding of the Roman Sol than, for example, the Syrian Jupiter Dolichenus is for our understanding of the Roman Jupiter.
Read more about this topic: Sol Invictus