Complex sentence: The power had only been maintained under Selim II by the genius of the all-powerful grand vizier Mehmed Sokolluwho remained in office until his assassination in October 1579.
Split into simpler sentences: The power had only been maintained under Selim II by the genius of the all-powerful grand vizier Mehmed Sokollu. For, though Sokollu remained in office until his assassination in October 1579.