Complex sentence: During the period of their suzerainty over the Caucasus, the Sassanids made attempts to promote Zoroastrianism there with considerable successes, and it was prominent in the pre-Christian Caucasus (especially modern-day Azerbaijan).
Split into simpler sentences: During the period of their suzerainty over the Caucasus, the Sassanids made attempts to promote the religion further in their territories, with relatively considerable successes. Zoroastrianism was prominent in the pre-Christian Caucasus region (especially modern-day Azerbaijan).