Imagine a roman legion from Ceasar Army which disappears and is transported to an alternate world, which looks mostly as the Mediterranean world around 600 AD. In this world the strongest and richest (although not exactly dominant) power is Empire of Videssos, which is actually a mirror image of Byzantin Empire in VII century AD (with some minor anachronisms), except that the magic in this world is real.
The misplaced legion becomes a precious mercenary force in service of Videssos and is soon put to the test of many wars waged by the powerful but vulnerable empire. The leader of the legion, Marcus Scaurus, attracts also the personal hostility of a horribly malevolent and greatly powerful warlock (no more details here to avoid the spoilers) and there is no certainty that he can survive this challenge.
The master of alternate history, Harry Turtledove, is also a professor of history and his speciality is the Byzantine empire - and it shows very much in this book. The ennemies (Makurans, equivalents of Persia and Ksatrish, the steppe nomads) and allies (Vaspurakaners, who are the local version of Armenians and Halogans, who are the equivalent of the Varangian guard) of Videssos are also very well described.
This is a very well written, original, smart and very erudit fantasy book, with many surprising twists in the action. I enjoyed it greatly and I recommend it warmly.