Crusader Kings 2 is now free (opens in new tab), which is nice. And so is The Old Gods expansion, but only if you sign up to the Crusader Kings newsletter (opens in new tab). You'll get a confirmation email and, pagan deities willing, then receive a Steam Key for the DLC.
The Old Gods is about paganism, notably adding the Viking faction, but also shifting the start date back to 867. This way you can play as the dynasty of Alfred the Great founding the Kingdom of England, but you can also be a West African tribe. While ranking the best Crusader Kings 2 DLC (opens in new tab), Tom Hatfield called it "hands down the best expansion, and the first you should buy." Meanwhile, Steam user Tovarishch says, "10/10 would leave father's corpse on a hill to be eaten by vultures and marry attractive 16 year old sister again".
Those are both recommendations, in their own way.