Complex sentence: Oda and Mieszko I, ruler of the Polans, were married in 980; they and their sons appear as "Ote and Dago (me)" in a document known as the "Dagome iudex".
Split into simpler sentences: Oda and Mieszko I, ruler of the Polans, were married in 980. Oda appears as "Ote and Dago (me)" in a document known as the "Dagome iudex".