Complex sentence: Odo held the county of Serimuntgau which Thietmar I of Meissen, a known son of Hidda, had held and he was buried in Nienburg, a monastic foundation of Thietmar's.
Split into simpler sentences: Odo held the county of Serimuntgau which Thietmar I of Meissen, a known son of Hidda, had held. He was buried in Nienburg, a monastic foundation of Thietmar's.