Paradox Interactive's convention, PDXCON, kicks off very shortly, and to celebrate the publisher is offering Crusader Kings 2 for free on Steam. It's not a limited time demo, so if you grab the game from now until (presumably) the end of PDXCON on October 20, you'll get to keep it forever.
Of course, the base Crusader Kings 2 game is barely scratching the surface of what's in the game nowadays: there's a frankly intimidating amount of DLC for the grand strategy game, but if you're keen to jump aboard in full measure, all of the major DLC bundles are at least 50 percent off at the moment.
Paradox is also holding a big sale on Steam, with recent titles like Age of Wonders: Planetfall and Imperator: Rome getting discounts of 10 percent and 33 percent respectively. The back catalogue stuff is where the really steep discounts are, though: Prison Architect, Stellaris and Europa Universalis IV are all 70 percent off, for example. Check out the full sale here.