Opus Gemini III by Romanike

Around 70 B.C., a tradeship heading for Rome foundered off the coast of Antikythera.
On board was the most sophisticated device known to the classical world: The Antikythera Mechanism - a set of gears that might foretell the ways of the stars. In the sea, the Antikythera Mechanism escaped the purposeful destruction that hit its siblings a hundred years later.
Save for one: The OPUS GEMINI, that has been ruined only twenty years ago. Princess Iulia Balbilla is still striving to retrieve it, so that she may restore her power over hearts and minds of the masters of Roma.
But one agent of Commagene, Pernica the Swift, has finally learnt about the unhappy fate of the OPUS GEMINI. And Balbilla is determined that Pernica shall pay her wavering with her life.

Its sole surviving sibling is now in the hands of Catvalda, Supreme Father of Mount Dounobriga. Currently he is inviting all the barbary lands to unite under his emerging power. At his side is Iulius Adrianus, out of reach beyond the Limes. To find him, Pernica has to return into her past and confront her most dreaded enemies in the heart of their might: the Germani.
And Adrianus has received evidence that even the evil Headhunters are active on far Mount Dounobriga.

The Dominion Device may bring a new power to Germania – and doom over the Roman Empire.