The Wicked Day, published in 1983, ten years after the last "Merlin" book, relates Mordred's story.
This is not part of the Merlin trilogy, or quadrogy or whatever a 4-book collection is called. It's written in a different style, and a 3rd person narrative that looses some of the immediacy of the Merlin books, but is still effective.
Lady Stewart, unique among modern writers (and I'm including here Geoffrey of Monmouth, T. H. White, Thomas Mallory, etc) turns Mordred from a hideous monster into a human being, as he was in the very early cycles of legend. She also explores the beautiful Orkney Islands, and Morgause and her wild brood (Gawain, Agravaine, Gaheris, and Gareth).
Once again, we have a strong (twisted and evil, but still strong) female character in Morgause, who finally comes into her own. Morgana la Fey and Nimue/Niniane also appear, but are less fully developed.
This is a great book, and an excellent interpretation of Mordred's Legend, as well as an exploration of Morgause's other boys.
The book lags a bit towards the end, although the ending is solid, and Guenevere remains pretty undeveloped.