Misplaced Legion
by Harry Turtledove

The legion was far from Caesar’s Gaul and Rome, banished to a world of magic and dark sorcery.

Book One of The Videssos Cycle

As they faced one another in a duel of survival, the Roman tribune Marcus Scaurus held the spell-scribed sword of a Druid priest, and the Celtic chieftain Viridovix held a similar sword, bespelled by a rival Druid sorcerer. At the moment they touched, the two found themselves under a strange night sky where no stars were familiar and where Gaul and Rome were unknown. They were in an outpost of the embattled Empire of Videssos—in a world where magic and dark sorcery would test their skill and courage as no Roman legion had ever been tested before.