If you never got around to playing Assassin's Creed IV: Black Flag, then you missed out on what I'd argue is the best Assassin's Creed game in the series. But today's a good day to try it out: It's now available for free on PC.
Ubisoft is holding a promotion this month to celebrate the tenth anniversary of the launch of the first Assassin's Creed: it's giving away PC downloads of Black Flag. From now until December 18, you can get the game through Ubisoft's Uplay service. And once you claim the game, it's yours to keep.
Although this year's entry in the series--Assassin's Creed Origins--is, by all accounts, a good game, Black Flag is still considered by many fans (and myself) a high-water mark of the franchise. Black Flag added a deep naval combat system, giving you a pirate ship and the ability to plunder privateers and other navies. It's well worth checking out, especially since it doesn't cost anything.
Ubisoft also put out a standalone story expansion to Black Flag, called Freedom Cry. It's a great add-on that explores more about the life of Black Flag's side character, Adewale. If you're a PlayStation Plus subscriber, you may have downloaded Freedom Cry for free on PS4 a few months ago--otherwise, you can buy it for $15 on PC through Steam or Uplay.
https://free.ubisoft.com/promotions/assassins-creed-black-flag/16/