Complex sentence: In the 17th century lots of Bulgarians especially in the Rhodope region were forced to become Muslims against their will and thousands of those that declined were ritually killed by the Bashi-bazouk Ottoman soldiers in front of other Bulgarians to watch.
Split into simpler sentences: In the 17th century many Bulgarians, especially in the Rhodope region, were forced to become Muslims against their will. Thousands of those who declined were ritually killed by the Bashi-bazouk Ottoman soldiers, in front of other Bulgarians forced to watch.