Complex sentence: The Deathwatch are uniquely organized in that the Marines that make up the Deathwatch are drawn from many different Chapters; specially trained alien fighters that are sworn to serve a term with the Deathwatch before returning to their Chapter of origin.
Split into simpler sentences: The Deathwatch are uniquely organized in that the Marines that make up the Deathwatch are drawn from many different Chapters. They are then specially trained to counter Xeno threats and are sworn to serve a term with the Deathwatch before returning to their Chapter of origin.