Complex sentence: Theophylact however lived and wrote during the reign of Heraclius the Younger, and is strongly biased in favour of his father, and his account is suspect of exaggerating or even inventing Comentiolus' cowardice with the aim of glorifying Heraclius.
Split into simpler sentences: Theophylact however lived and wrote during the reign of Heraclius the Younger, and is strongly biased in favour of his father. His account is thus suspect of exaggerating or even inventing Comentiolus' cowardice with the aim of glorifying Heraclius.