While Crusader Kings 2 is now permanently free on Steam, the DLC is not. However, until March 9 you can grab the Black Death expansion The Reaper's Due for zero dollars and zero cents if you've already got the base game in your account.
The Reaper's Due has a variety of ways you can deal with the plague, like building hospitals or quarantining your castle while the peasants die outside your doors. Oddly enough given the theme it's one of the siller DLCs, as it also adds the ability to play chess with Death, seek immortality, and have a pet cat.
The third game in the series is due out this year. Here's everything we know about Crusader Kings 3.