Complex sentence: It is generally thought that the Tribal Hidage originated from Mercia, which held extensive hegemony over the Southumbrians until the start of the 9th century, but some historians have proposed that the text is of Northumbrian origin.
Split into simpler sentences: It is generally thought that the Tribal Hidage originated from Mercia, which held extensive hegemony over the Southumbrians, up to the start of 9th century. Alternatively, some historians have proposed that the text is of Northumbrian origin.