Complex sentence: An invasion army under Konrad conquered the island of Gotland in 1398, destroyed Visby, and drove the Victual Brothers out of Gotland and the Baltic Sea, the Monastic State of the Teutonic Knights in Prussia was at the peak of its power.
Split into simpler sentences: An invasion army under Konrad conquered the island of Gotland in 1398, destroyed Visby, and drove the Victual Brothers out of Gotland and the Baltic Sea. The Monastic State of the Teutonic Knights in Prussia was at the peak of its power.