Complex sentence: Rather than remarrying, Schwarzenberg entered the Johanniterorden, becoming "Herrenmeister" ("Lord of the Knights", or Grand Master) in 1625, retaining that office until his death, sixteen years later.
Split into simpler sentences: Rather than remarrying, Schwarzenberg entered the "Johanniterorden", becoming its "Herrenmeister" ("Lord of the Knights", or Grand Master) in 1625. He retained that office until his death, sixteen years later.