Complex sentence: He was still at the monastery when Heraclius overthrew Phocas in 610, and was sent by the new Byzantine emperor to negotiate with Phocas's brother, Comentiolus, who commanded the eastern army.
Split into simpler sentences: He was still at the monastery when Heraclius overthrew Phocas in 610. The new emperor recalled him and sent him to negotiate with Phocas's brother, Comentiolus, who commanded the eastern army.