Is Destiny 2: The Witch Queen free?

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.