One of our favourite strategy games of recent years is undoubtedly Crusader Kings II, the dynasty simulator from Paradox that puts players at the head of a medieval family and has them steer their way through backstabbing politics, arranged marriages, bitter wars, religious expansion, and so much more. The game has been heavily supported over the years via a number of expansions, and so Paradox is no doubt hoping that by giving away the base game, players will dip into the extensive back catalog of DLC upgrades.
Even if you stick to the base game, CK2 offers a deep and engaging experience full of player-generated stories and entertaining accidents, and seeing as how the game is being given away for free, if you're inclined towards strategy games we'd recommend you pick it up and give it a try before the offer ends tomorrow (April 7 at 6pm BST / 7pm CEST). It's also worth noting that all of the expansions for the game are discounted too, so even if you've got the base game, now would be a good time to grab that DLC you've been contemplating.