Crusader Kings II is free on Steam, and if you pick it up right now you’ll be able to keep it forever. Just head over to the Steam store page and activate the game to your account. You don’t need to download it right away to keep it. Crusader Kings II was released in 2012 by Paradox Interactive and currently holds a 77% “Mostly Positive” rating on Steam. As of this posting, there are nearly 30,00 people playing concurrently.
Granted, you’ll still need to throw down a pretty penny to get every one of the game’s DLC packs, which are currently on sale at 50% off.