https://www.pcgamer.com/crusader-kings-2-is-free-on-steam-and-you-can-keep-it-forever/

Crusader Kings 2 is a medieval grand strategy game that has players vying to establish and expand their kingdoms across multiple generations during a time of great political and military strife. Those who like it, like it a lot, but it's not the most accessible thing ever, which has likely kept quite a number of more casual strategy fans away from it. But for the next 48 hours it's free on Steam, and if you grab it during the giveaway it's yours to keep forever. 

To get it, just click the "Install Game" button, which will register CK2 to your account and begin the install. (The download is about 1 GB, but you can opt to abort the install if you don't want to dive into it right away.) While you're there, you might also take note of the plethora of DLC that's available, all of which is currently on sale for half price except for Jade Dragon, the most recent of the bunch, which brings China to the fight and is 33 percent off.