The Temple and the Stone by Katherine Kurtz, Deborah Turner Harris is one of Open Road Media's daily deals for $1.99 today.
This is not from the Deryni series which I recommend (see below), but seems to be related to the Adept series the two authors wrote together.
https://www.kobo.com/us/en/ebook/the...nd-the-stone-2
https://www.amazon.com/Temple-Stone-.../dp/B01FRQEV8S
During the fight for Scotland’s independence, the mystical Order of the Knights Templar battles ancient evil and a treacherous king in this gripping alternate history.
In case you haven't got it previously The Legends of Camber of Culdi Trilogy
(Camber of Culdi, Saint Camber, and Camber the Heretic) by Katherine Kurtz is currently still $2.99
https://www.kobo.com/us/en/ebook/the...-culdi-trilogy