As my first ATL I wanted to attempt something ambitious (at least by my standards), mainly a Dark Ages Europe that is still pagan/polytheist. So, without further delay, here are the main PODs/background events that will bring you closer to understanding the timeline I will create. I hope that they are plausible enough.
POD1: Neither Jesus nor John the Baptist is born. Due to this it can be assumed that no Christianity-esque religion has been formed so far.
POD2: Instead of having a Christian vision prior to battle, Constantine has a vision of the god Sol Invictus and proceeds to defeat his enemies, ultimately becoming the sole Roman Emperor in 1077 a.u.c (324 CE OTL). As a part of the fulfillment of his vision, Constantine begins to believe that he is an incarnation of the sun god and styles himself as the Sun King.
POD3: Instead of choosing Byzantium to be his new and improved capital as he did in OTL, Constantine chooses Thessalonica due to its harbor and position on the Via Egnatia. He renames the city Solarianople in honor of his self-styled title and greatly expands the city’s infrastructure and aesthetic beauty, as well as laying the foundations for both a palace and a temple complex to Sol Invictus.
POD4: Constantine does not have his first-born son, Crispus, killed. Instead, when Constantine dies in 1090 a.u.c. (337 CE OTL), he gives Crispus control of the Eastern Roman Empire as the next Sun King with his other sons dividing up the western half of the empire as co-Augusti of the West. This marked the last year in which the two halves of the empire would be united.
POD5: Of the three sons ruling over the western half of the empire, only Constantius II is still alive by 1103 a.u.c. (350 CE). When he dies ten years later without having named a successor, the Western Roman Empire’s Senate names Marcus Honorius, an up and coming general who had served under Constantius II, Augustus of the West. Honorius proves capable of holding together the failing empire. However, when he dies in 1125 a.u.c. (373 CE), his son, Marcus Justus proves incapable of doing what his father managed to do and by 1153 a.u.c. (400 CE) the barbarians are pressing in on the Western Empire’s borders.