The character meets a witch of some kind that sucks the life from men, but manages to escape. He first encounters her when she seems to be running from some kind of creature or guards. He helps her escape, but it turns out it was a trick. Later, and the most vivid thing I remember is how magic is kind of iffy in the book, but the main character meets a "demon" which is basically a spark of light and he commands it through physics lingo. I know it's vague, but I can't remember much more, other than the witch ends up becoming a nun and the fortress where the nuns are is assaulted by some kind of magic that makes them question their faith or jump off the wall. The women when they lost their faith would go to the cathedral and pray. Thanks again for your help!
This would be Christopher Stasheff's A Wizard in Rhyme series, specifically the first book, Her Majesty's Wizard, which features a linguist brought to the world of Merovence, an alternate Europe where there is no English channel and magic exists. Words, specifically poetry, create magic, and our protagonist arrives with centuries of literature which he can adapt and is thus a mighty wizard in their world. The witch turned nun is Sayeesa, who joins the House of Cynestria. One of his companions is a Maxwell's Demon, a "spirit of perversity", who can do anything which involves breaking the natural laws of physics.