Duncan had to return.
Because during this time, a major event occurred that was enough to affect the entire European situation.
Attila the Hun, known as the God's Whip, led his army to split into two routes, bypassed Paris, and attacked the central Gaul stronghold of Oleronia (Orléans), intending to conquer the Franks in one fell swoop, defeat the Empire, and bring the Franks completely under his control.
As mentioned before, there were two groups of Franks: one submitted to the Hunnic Empire, the other favored the Roman Empire.
So when Duncan was traversing the European continent, he saw two different Frankish forces fighting each other, with a small number of elite Hunnish cavalry present. This time, the God's Whip was serious; he personally led his army westward, preparing to smash the declining Western Empire.
"The Battle of Chalons is about to erupt."