It’s completely free if you own Fallout 4, like Enderal for Skyrim.
It’s a mod that effectively makes Fallout 4 a new game. It’s set in its own world, has its own story, and has its own unique mechanics.
Some people are even saying it’s the best Fallout since NV.
Edit: If you’re curious here’s a link to the site. You can grab it easily from GoG if you own Fallout 4 over there, it’s a bit more involved if you have the Steam version.