https://en.wikipedia.org/wiki/Civil_wars_of_the_Tetrarchy#War_of_Constantine_and_Maxentius
Maxentius organized his forcesโstill twice the size of Constantine'sโin long lines facing the battle plain, with their backs to the river. Constantine's army arrived at the field bearing unfamiliar symbols (the "Chi-Rho") on either its standards or its soldiers' shields. According to Lactantius, Constantine was visited by a dream the night before the battle, wherein he was advised "to mark the heavenly sign of God on the shields of his soldiersโฆby means of a slanted letter X with the top of its head bent round, he marked Christ on their shields." Eusebius describes another version, where, while marching at midday, "he saw with his own eyes in the heavens a trophy of the cross arising from the light of the sun, carrying the message, In Hoc Signo Vinces or "Conquer By This"; in Eusebius's account, Constantine had a dream the following night, in which Christ appeared with the same heavenly sign, and told him to make a standard, the labarum, for his army in that form. Eusebius is vague about when and where these events took place, but it enters his narrative before the war against Maxentius begins. Eusebius describes the sign as Chi (ฮง) traversed by Rho (ฮก), or โง a symbol representing the first two letters of the Greek spelling of the word Christos or Christ. The Eusebian description of the vision has been explained as a "solar halo", a meteorological phenomenon which can produce similar effects. In 315 a medallion was issued at Ticinum showing Constantine wearing a helmet emblazoned with the Chi-Rho, and coins issued at Siscia in 317/18 repeat the image. The figure was otherwise rare, however, and is uncommon in imperial iconography and propaganda before the 320s.