Complex sentence: An estimated 15--20,000 Syriac Orthodox Christians lived in northern Iraq in 1991, gradually moving there from Tikrit from a period between 1089 and the 1400s due to persecution and a genocide by Timur.
Split into simpler sentences: An estimated 15--20,000 Syriac Orthodox Christians lived there in 1991. The Syriacs of the Nineveh Plains originate from Tikrit, but immigrated to the north during a a period between 1089 and the 1400s due to persecution and a genocide by Timur.