Complex sentence: After The Great Migration of Serbs in 1689, led by patriarch Čarnojević, the big part of Serbian people came to Austria, which was trying to catholicize those Serbs, and make them write using the Roman alphabet instead of their traditional Cyrillic.
Split into simpler sentences: After the Great Migration of Serbs in 1689, led by the patriarch Čarnojević, the largest part of the Serbian people came to Austria. There they came under pressure to become Catholics, and to write using the Roman alphabet instead of their traditional Cyrillic.