Complex sentence: Jacob Svetoslav was close to the Bulgarian court and pledged loyalty to Constantine, who made him the ruler of a domain usually considered to have been south of the Vidin region in the west of the Bulgarian Empire.
Split into simpler sentences: Jacob Svetoslav was close to the Bulgarian court and pledged loyalty to Constantine. The tsar made him the ruler of a domain usually considered to have been south of the Vidin region in the west of the Bulgarian Empire.