CRUSADER KINGS 2! This is one of the best games ever made, and this deal is completely insane (and available for the next 10+ days):
https://www.humblebundle.com/games/crus ... s-2-bundle
CK2 is a grand strategy game that is more or less a medieval dynasty simulator. The learning curve is steep, and it's not flashy in terms of graphics or interface . . . but oh my god is it addictive and replayable. The $15 deal above gets you the base game and all the key DLC for a fraction of the price.
I give CK2 my highest possible recommendation.