Complex sentence: She married Æthelred, Lord of the Mercians, who had accepted Alfred's overlordship; he ruled western Mercia at a time when the eastern part was under Danish Viking rule.
Split into simpler sentences: She married Æthelred, Lord of the Mercians, who had accepted Alfred's overlordship. He was the ruler of the western part of Mercia at a time when the eastern part was under Viking rule.