Crusader Kings 2 is newly made free to play strategy wargame, and so is the unique expansion of the Old Gods, but only if you sign up to the newsletter of the Crusader Kings. You’ll typically obtain a confirmation email and then receive a Steam Key for the DLC.
The Old Gods traditionally speak of paganism, introducing the Viking faction, but also moving the start date back to 867. This way you can play as Alfred the Great’s dynasty that formed The great Kingdom of England, but you can additionally be a West African tribe. While justly ranking the best Crusader Kings 2 DLC, Tom Hatfield called it “hands down the best expansion, and the first one you should buy.” Meanwhile, Steam user Tovarishch said, “10/10 would leave father’s corpse on a hill to be eaten by vultures and marry attractive 16-year-old sister again”.