If you are playing on PS4 then you will also need to download the relevant compatibility packs afterwards. Again, you will be prompted to do this. Destiny 2: The Witch Queen costs $39.99. Meanwhile, the Season of the Risen content is available totally for free.
Do you have to buy Destiny 2: The Witch Queen?
No. You can buy Witch Queen on its own and enjoy everything it has to offer. This should include a campaign, the Savathun Throne World location, the new raid, and any other unannounced secrets.