Answer;
-The Mongols
-Herodotus
-Persian empire, which became the model for pretty much all land-based empires throughout the world. Except for the Mongols.
Much of what we know about the Persians and their empire comes from an outsider writing about them and one of the first true historians was Herodotus, whose famous book The Persian Wars talks about the Persians quite a bit. Now the fact that Herodotus was a Greek is important because it introduces us to the idea of historical bias.