The action begins in A.D.62 (+/-).
At this time there are no elves, dwarves, 1/2lings, gnomes, tieflings, dragonborn, etc. Maybe just stories of them in far off corners of the Empire or beyond its border. But not as playable races.
Divine & Arcane magic is not a thing.
The west coast of Italy is rocked by a series of heavy earthquakes over the course of several days.
Although more violent than usual (causing major damage to various towns/infrastructure/etc - that takes years to repair) , nothing much is really thought of the incident. Afterall, the areas always been subject to quakes and minor volcanic activity....
Some time after that (several months?) the "fun" begins.
Tales begin circulating of strange creatures roaming the country side.
At first this is laughed at. But but eventually the problem has to be admitted & a Legion of troops gets sent.
Monsters are confirmed to be REAL!
Meanwhile followers of the old gods (remember, this is only 63 A.D., but let's pretend Christianity spread a bit faster for plot purposes) start to manifest spell casting capabilities & other class powers.... Almost as if the old gods are aiding thier faithfull.
Multi-classing, re-training, & wholly new characters become options at this stage.
This continues for a # of years, increasing in scope.
Squads are sent to recover ancient Greek etc artifacts, consult oracles, etc.
Tieflings begin to be born in western Italy. They mature unnaturally fast....
Troops sent to remote ends of the Empire/map DO run into elves, dwarves, & such. Non-human PC races become options.
And then, in 79 A.D. all Hell - or more accurately Hades - breaks loose as the titans claw their way back into the material world through Mt. Vesuvius.
Monsters, Magic, & Gods have officially returned to the Roman Empire.