Complex sentence: Geoffrey of Monmouth includes Constantine in a section of his "Historia Regum Britanniae" adapted from Gildas, in which he makes the reproved kings successors, rather than contemporaries as in "De Excidio".
Split into simpler sentences: Geoffrey of Monmouth includes Constantine in a section of his "Historia Regum Britanniae" adapted from Gildas. In his usual manner, Geoffrey manipulates his source material, recasting the reproved kings as successors, rather than contemporaries as in "De Excidio".