Cast:
Milo is now Derek Hale
Aurelia is now Lydia
Cassia now Cassius (Stiles' Roman name)
Atticus is now Boyd
Ariadne is now Claudia (Cassius' mother)
Severus is now Noah (Cassius' father)
Graecus is now Christopher (no relation to Gerard in fanfic)
Corvus is now Gerard Argent
Milo's Mother is now Talia Hale
Proculus is now played by one of Gerard's loyal Hunters
Milo's Father is now Derek's father
Felix is now Harris

Crossover Summary:

In 79 AD, the Hale Clan*, a Celtic tribe of horsemen is slaughtered by the Roman Senator Gerard, his right-hand man Proculus and their army. A little boy, Derek, is the only survivor that is captured later and sold as a slave. Seventeen years later, the slave Derek turns into an invincible gladiator in a province and is brought to Pompeii to participate in the games in the arena. While walking to Pompeii, the noble Cassius and his chaperone Lydia cross the path of the slaves and Cassius is fascinated by Derek. Derek shares the cell of Boyd, who is near to get his freedom depending on winning his last fight. Meanwhile, Cassius meets his parents Noah and Claudia and learns that he has been betrothed to the corrupt Senator Gerard who is pressing his parents to marry him. During the games, the Mount Vesuvius erupts and Derek and his friend Boyd succeed to escape from the arena. But Derek wants to save his beloved Cassius in the middle of the chaos and the Romans.

* Since the Hales were/are werewolves in Teen Wolf, in order to maintain a connection to wolves, I've made it so the warriors and chiefs of the Hale Clan would often get tattoos when they came of age depicting various wolves as wolves are the symbols of the Clan's Patron Gods: Cernunnos, the Celtic Horned-God of the animals and Cerridwen, the Celtic Goddess of the Moon and fertility.

Try listening to the Pompeii soundtrack by Clinton Shorter when reading this.