Asmodeus J-for John Jones is Summoned by Nevianne the Witch to combat the evil Palegos, who's oppressing the countryside. She's expecting someone nine feet tall, with fangs, and she gets him. He not only doesn't believe in magic, he suppresses its use for a mile in any direction. There's no way she can send him back. He's capable of slipping between reality streams, but he's so far away he can't find his way back.
How far away? Nevianne and her coven live in the Really Later Roman Empire. There was no battle at the Milvian Gate, no "In Hoc Signo Vinces." Emperor Julian reigned for thirty two years, not three, and he declared parity among religions, establishing a Ministry of Religion, to enforce it. Nevy and her coven are pagans. Athaulf the Visigoth king wasn't murdered in his bath. He reigned with Galla Placidia as Emperor Athaulf I. There was also an Emperor Genseric. There's a synagogue in Flumen Martii, the capital of Agus, which covers Maryland, Delaware, Pennsylvania, and part of New York. Asmodeus learns magic from the Archbishop of the Church of Saint Simon Magus.
Under Roman rule North America was settled around 1000 A.D. The population of Agus is about a third native, a third Saxon (and Celtic Briton and some Dane), and a third Latin. The official language is Latin. I don't mention it in the text, but there's also an official Latin Academy and an official Latin language, so there's no official French or Italian or Romanian; there are just different accents. The natives in Nevy's area speak Lenape, and there are Iroquois living to the north. The Susquahannocks aren't extinct. Nor are passenger pigeons.
Nevianne and Asmodeus fall for each other immediately, and hard. She first saw him as her future husband in her first vision when she was nine. Demons can be summoned only by maidens. Once they complete the task, the maiden is theirs forever. Nevy expected to be a human sacrifice to the demon. She and Asmodeus get to mess around a lot, but she has to remain a maiden at least until the wizard is vanquished.
As soon as Asmodeus (he prefers Jack) is Summoned, Palegos sends Nannakussi, his Lenape minion, and his men to kill the entire coven. Blaeda, Nevy's best friend and a member of the coven, who's a Seeress, warns them. Jack constructs an IED from the witches' gunpowder and flour, with rocks for shrapnel, and kills all of Nannakussi's men, burning Nevy's house down in the process. He conks Nannakussi in the head with a chunk of cord wood, defeating him in personal combat. Nannakussi and his wife and five-year-old daughter become Jack's slaves by law. They were Palegos' slaves before.
Besides the witches and the archbishop, there are flying monkeys, monkey-faced bears, a precocious five-year-old witch, a fire-farting imp, and a fight to the death between Asmodeus and a real demon.
It's rip-snortin' adventure with a language problem.