Complex sentence: The Mongol Empire--being at its height the largest contiguous empire in history--had a lasting impact, unifying large regions, some of which (such as eastern and western Russia and the western parts of China) remain unified today, albeit under different rulership.
Split into simpler sentences: The Mongol Empire--at its height the largest contiguous empire in history--had a lasting impact, unifying large regions. Some of these (such as eastern and western Russia and the western parts of China) remain unified today, albeit under different rulership.