The Pendragon Cycle by Stephen R. Lawhead

I've always been a sucker for Arthurian legend and so far I have been most impressed with this series of novels that were published in the 80s and 90s. The main series takes place in the latter days of Roman Britain and combines real history as well as fantastical and religious elements to tell the story. There are five novels in the main series plus a sequel novel which takes place in modern times that wraps the whole story up. I'm about halfway through the second novel (entitled Merlin) and have to say that I am hooked. The first novel was called Taliesin and follows Merlin's grandfather, father, and mother - two men of early English nobility and an Atlantean princess. The third, fourth, and fifth novels appear to revolve almost completely around King Arthur.

The series is likewise currently being adapted into a streaming series by The Daily Wire.