Complex sentence: It was one of the most revered icons in the monastery, attracting generous donations from the reigning monarchs (first Ivan the Terrible, then Boris Godunov and his family), although the main object of worship in the monastery was the Sergius of Radonezh's relics.
Split into simpler sentences: It was one of the most revered icons in the monastery, attracting generous donations of the reigning monarchs (first Ivan the Terrible, then Boris Godunov and his family). The object of worship in the monastery was the Sergius of Radonezh's relics.