The challenge is to memorize the largest 10-25 empires that existed in the world. This can be just the empire names, or with added challenges like, time period empire existed, population the empire reached, land area the empire occupied, leaders of the empire, and/or GDP size.
This information can be found in this Wikipedia article: https://en.wikipedia.org/wiki/List_of_largest_empires
And here are the top 10 biggest to get you started:
French Colonial Empire