Hey, so to be able to get a key for this DLC : the old gods
You must go to Crusader Kings website and subscribe to their newsletter to get "The Old Gods DLC". To do that, you have to log in or create an account on Paradox Interactive website.
Then check your email and confirm your subscription.
After, Visit the accounts page and click manage to retrieve your Steam code for Old Gods.
And finally add it to steam
(Note you must have the game Crusader Kings 2, which is free on steam to be able to add the DLC, sound obvious but I rather say it)
Available until they run out of keys.
Hope this is detailed enough to be easy to get the key ! Feel free to add more tips in the comments if I'm unclear.