From the publisher's website, with small user summarization to make make setting rather than product specific:
Roma Imperious is where Alternate History meets Roman Fantasy. During the rule of Constantine, pressed to defeat Maxentius at the battle of the Milvian Bridge, he turns to a group of ambitious druids. They perform a ritual, calling on their gods to give victory to Constantine. In exchange for the spell, Constantine promises freedom for the peoples of western Europe. He would not keep that promise and further he would demand their secrets. The Druids, over the next four hundred years, would rise against Roma demanding their due.
Meanwhile, Rome would follow its own path, building and researching magic in their own way. As the world changes, magic becomes more common and the barbarians flounder in ancient rituals the Romans would push forward into newer and more reliable forms of magic.
The setting for Roma Imperious is one of the world but centered on Roma. The Jade Empire is an empire of the far east taking up what is modern day China and some of southeast Asia. It is ruled by a Han Emperor who discovered the secret of immortality. He has ruled for nearly six centuries and brought a peace and wisdom to the Jade Empire not seen before. The Jade Emperor, a title taken by him early on, see the Romans as a direct threat.
Skandia is the pre-Viking Norse who have been forced to find other routes of expansion as faced by an increasingly magic reinforced Empire to the south and east. They have taken their ships and magic and begun to spread to the west. They now have colonies in the western hemisphere where they face demons they have brought with them and native they never expected to meet. The Skandians use their magic to aid their voyages of exploration but more importantly, to serve their gods. Odin, Thor, Freyr and many of the other Norse gods live amongst them. Day to day, the gods direct them and grant their priests a bit of their power. This aids them in their battles against the Trold, giants and the most feared Manes Atrox.
Alkasas is a kingdom of evil wizard-priests who occupy what would be northern Russia today. Alkasas is ruled by a king but, as with the Norse, their gods reside in the temples and cities of the kingdom. Hetam-munnun is the goddess of death and rebirth and her priests are a strong political force in the kingdom. Although she does not lead the gods her zeal for the Final Battle where the fate of the world will be decided through a test of arms drives here to the fore. She sees the Roman armies as the single greatest prize to attaining her goals. This puts Alkasas in direct conflict with Roman rulers, often leading to war and intrigue.
Roma Imperious is a collection of all these elements. The Romans are often defined by those outside their boundaries as much as the people they rule.
The Romans measure their calendar from the founding of Rome in 753 B.C. The current year in this alternate setting is 1461 A.U.C. or 708 A.D.