Complex sentence: Integrated in the Obodrite state during the following decades, the Kessinians lost their discreteness, and were assimilated by German settlers during the medieval Ostsiedlung, who turned the Obodrites'state's successor, the Duchy of Mecklenburg, into a German region during the 12th century.
Split into simpler sentences: Integrated in the Obodrite state during the following decades, the Kessinians lost their discreteness. Finally, the Kessinian remains were assimilated by German settlers during the medieval Ostsiedlung, who turned the successor of the Obodrites' state, the Duchy of Mecklenburg, into a German region during the 12th century.