Complex sentence: Mary Stewart's first two books in her Arthurian saga, "The Crystal Cave" and "The Hollow Hills", feature Uther Pendragon as Merlin's uncle, Merlin being his brother Ambrosius' illegitimate son.
Split into simpler sentences: Mary Stewart's first two books in her Arthurian saga, "The Crystal Cave" and "The Hollow Hills", feature Uther Pendragon. Here, he is Merlin's uncle, since in this version the latter is his brother Ambrosius' illegitimate son.