Complex sentence: Mamai (?-1380) was a powerful military commander, of Borjigin descent, in the 1370s of the Blue Horde, which ruled over lands in what is now the southern Ukrainian steppes and the Crimean peninsula.
Split into simpler sentences: Mamai (?-1380), of Borjigin descent, was a powerful military commander of the Blue Horde in the 1370s. The Horde controlled the lands in what is now the southern Ukrainian steppes and the Crimean peninsula.