Is this a run-on sentence?
In 1227, Genghis Khan, the Mongol leader who conquered much of Central Asia, died he was buried in an unmarked grave, and its location remains a mystery today.

 - no
 - yes
This is a run-on sentence. It is formed from two sentences run together, joined without punctuation.
In 1227, Genghis Khan, the Mongol leader who conquered much of Central Asia, died he was buried in an unmarked grave, and its location remains a mystery today.
Here is one way to fix the run-on sentence:
In 1227, Genghis Khan, the Mongol leader who conquered much of Central Asia, died. He was buried in an unmarked grave, and its location remains a mystery today.