Yes, the base game has been free for years on Steam.
But the DLC not, they gave some away for free over the years though.
What Is Crusader Kings 2? - A Brief Summary - YouTube
Beware of this game, it's truly addictive, my most played game on Steam more than 260 hours.
It's about managing your "house" or "bloodline", kinda like in Game Of Thrones, you play as a ruler, moment you die, you continue playing with your chosen heir (usually your eldest son).
So it's a grand strategy simulator. You can do whatever you want, but everybody in this game has an opinion about everyone and you have to try play the politics or war to monitor your family and make sure you keep the throne and expand your power. This seems boring, but it's very addictive to find ways to keep your house safe over the years and do whatever to keep the power and all your vassals and underlings in check.
TLDR: If you ever wondered what it feeled like ruling as a ruler in the middle ages, play this game. You can pick whatever country or province in europe (christian countries only in base game) and start going.
Just play it.