The Deryni are humans with (discussed only in a companion text, not in the novels) a genetic mutation that gives them abilities we would often place in the realm of psychic powers; these powers can (and are) inherited from one or both parents. Certain Deryni with an extra mutation (also inherited) can heal most injuries. The novels in are set, as I said, in Gwynnedd, a pseudo-historical England, where the Deryni are considered a separate breed; in some places and times, they are a ruling class, while in others they are a derided underclass determined to be damned from birth. During the times when they are derided, they are determined by the Church to be damned from birth, and found by a potion that affects Deryni much more than pure humans. Because of this, the Deryni in Gwynnedd are persecuted in place of Jews and others of religions that the Church does not approve of - both because they are much easier to find, and because their ancestors usurped the throne of Gwynnedd and then were, in their turn, overthrown.
In addition to the inborn, almost instinctive "psychic" powers, the Deryni also have the potential to become ritual magic workers - expanding their abilities to things like scrying, taking the seeming of other people, lighting fires, using transporter-like devices fueled by magic to move between places, weather magic, and related events.
The characters in the series are people you'd want to - except for the ones you want to kill. As I said, the original trilogy (which is now in the middle of the timeline, as Kurtz wrote books that took place both before and after the original set) is a little rough, but the characters are wonderful, and her writing improved noticeably the more she wrote - I would suggest reading them in the chronological order in which they occur, rather than in publication order. The Wikipedia article is actually pretty good, and the related article on the novels gives the chronological sequence.
These novels were originally recommended to me over 20 years ago by a coworker, and I still have the original volumes I purchased, along with the additional ones that have been published since then. As Sukerkin said, characters will die who deserve to live, and vice versa - but it makes the stories more real, at least to me.
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